I assume this happens to everyone in the US, and I believe the reason is simply due to competition.
Here are my data points:
I was with Allstate for 17 years, and the rates were pretty consistent for a long time, then about 6 years ago every 6 months my rate would go up slightly.
After 2-3 years of that I called and I asked why?
I was told it was due to the average rates in my area increasing, etc.
I pointed out how loyal of a customer I was, and nothing helped.
So I got a quote from Geico which was $130 lower per 6 months.
I called AllState and asked them to match it
or I'm leaving.
They wouldn't.
So I switched to Geico.
1 month later my Allstate agent called me and told me she could match (or beat) the rate I had with Geico if I came back.
I said no. 2 years with Geico and my rates started inching up.
After 18 months of increases I went through the shockingly identical process, and ended up switching to StateFarm.
Now I've been with StateFarm for coming up on 2 years and it's happening again.
Note
I have never had a claim or a ticket in over 15 years.
My conclusion is that (all?)
car insurance companies in the US lose money on new customers, and they are willing to do that just to get the customer.
Slowly they raise rates to price them to where they should be.
The only way to get the lower rate is to switch companies every few years.
money.stackexchange.com/users/17718/TTT
Part of auto insurance is your potential liability cost.
I'm personally in the same boat as you, no infractions, same car, and my insurance went up (though less than yours).
I live in California, in an area where I'm likely to hit a Mercedes (or some other luxury thing) if I hit anything.
Lots of people in my area buy expensive cars and as a result I pay significantly more for the same liability coverage as I would in a lot of other zip codes.
Add to that normal inflationary pressure on auto-body repair shops and you have increases even if you, personally, don't present any new risk.
In response to the discussion below
I'm revising this point.
There are loads of factors insurers consider beyond your claims history, driving record and good student status.
Maybe a lot of people in your neighborhood are buying Mercedes or some other expensive vehicles.
Maybe your newly attained age triggers a different risk evaluation in their models.
Any number of criteria related to you could have changed apart from your personal driving record.
The carrier has a pool of people and the ability to price certain criteria related to you.
The insurer's overall pool needs to be profitable and sometimes that means higher risk people are subsidizing the cost of lower-risk people, and sometimes the opposite can be true.
Every part of the risk spectrum will generate claims, claim expenses aren't exclusive to the high risk segment of the pool.
When an insurer increases it's prices that means that relative to the expected claims of the pool, or some piece of your specific underwriting classification, it's not generating enough top-line revenue.
Maybe this price increase hits everyone in the pool, maybe it's only the higher-risk or the lower-risk people, maybe it's only the folks with big limit coverage, maybe the student discount gets smaller.
No matter the adjustment, the carrier has determined that it either can or should generate more revenue and/or shed some individuals from some facet of its pool.
No matter your personal driving record and perceived risk category, if you get an 8% increase you should go shopping.
It's likely that some other carrier wants someone in your underwriting category more than your current carrier does.
An 8% increase from your current carrier indicates they expect some number of people like you to leave.
money.stackexchange.com/users/22881/quid
Because it's profitable Insurance companies have found that if you raise peoples' premiums by 5%, a few of them will switch, but most of them either won't notice, or won't want to deal with the hassle and admin of switching in order to save a few %.
Net Result?
They have (a few) fewer customers = fewer payouts, and more revenue.
Hence more profits.
So they keep doing it.
If they lost more in revenue than they gained in higher premiums, they would stop doing it overnight.
But they don't, so they don't.
The same dynamics happen in (almost all) recurring-subscription service industries where people expect the prices to vary: Car insurance (in fact, most insurance), mobile phone contracts, rent, leases etc.
The only way to avoid it is to keep an eye on your bills and shop around as soon as they start going up.
money.stackexchange.com/users/24524/Kaz
Depending on where you live local regulations allow insurers to rate you based on many factors besides the type of car and how many accidents/tickets you've had historically.
Your rates might go up if you have recently moved, if your credit score has taken a hit, if you've become divorced, or if they have concluded that they have to raise all of their policies in your area by a fixed percentage due to a higher number of claims from your neighbors.
money.stackexchange.com/users/12089/NL - Apologize to Monica
As with many other industries, they are betting you will not consider the cost increase is worth the hassle of switching.
Loyal customers and well off customers are easy targets.
If they try to increase too high, you may be willing to switch.
If they increase too low, then they miss out on potential profits.
They probably have tested various amount increases and found a happy medium.
money.stackexchange.com/users/1794/James Lawruk
If you live in New York, and you took a driver safety course over three years ago , you can expect to lose the 10% mandatory car insurance discount that you would have been receiving.
Other states may have other discounts , I don't know all the details.
money.stackexchange.com/users/3216/zzzzBov
I have just received a new auto insurance bill for the next six months for my comprehensive insurance. It is about 8% higher than the previous six months whereas I have had no new moving infractions and the vehicle is the same as before, meaning older, meaning costing less to replace.  What actuarial reasoning could explain this increase?
Auto premium increasing with aging vehicle and no new tickets
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<united-states><car><insurance><car-insurance>
money.stackexchange.com/questions/75232
money.stackexchange.com/users/12359/amphibient
There are other factors that go into making up the price. They might increase everyone in your pool if there have been a lot of recent claims. It could also be to do with age or if your personal situation has changed. Auto insurance includes your potential liability cost and many factors are considered in addition to claims history, driving record and good student status. For example, your age may trigger a different risk evaluation in their models. When insurers increase their prices it means they are not generating enough top-line revenue relative to the expected claims of the pool. This price increase may have affected everyone in the pool, or maybe only the higher-risk people. Your rates might also go up if you have recently moved, if your credit score has taken a hit, if you have recently divorced, or due to a higher number of claims by your neighbours. Add to that normal inflationary pressure on auto-body repair shops and you have increases even if you, personally, don't present any new risk.  Regardless of your personal driving record and perceived risk category, if you get an 8% increase you should shop around for a better deal. Insurance companies have found that if they raise premiums by 5%, a few customers will switch, but most either won't notice, or won't want the hassle and admin of switching just to save a few %.  The only way to get lower rates is to switch companies every few years.
Auto insurance includes your potential liability cost and many factors are considered in addition to claims history, driving record and good student status. For example, your age may trigger a different risk evaluation in their models. When insurers increase their prices it means they are not generating enough top-line revenue relative to the expected claims of the pool. This price increase may have affected everyone in the pool, or maybe only the higher-risk people. Your rates might also go up if you have recently moved, if your credit score has taken a hit, if you have recently divorced, or due to a higher number of claims by your neighbours. Regardless of your personal driving record and perceived risk category, if you get an 8% increase you should shop around for a better deal. Insurance companies have found that if they raise premiums by 5%, a few customers will switch, but most either won't notice, or won't want the hassle and admin of switching just to save a few %.  The only way to get lower rates is to switch companies every few years.