Your legs aren't as sensitive to temperture extremes.
Right now it's winter here
and I'm walking around outside with a regular shirt, a wool sweater, and a wind breaker on my torso.
Inside
I take off the windbreaker an sweater.
However, inside or outside, I'm wearing the same single-layer pants and it's not a problem.
My legs don't feel hot inside or cold outside.
If I had that same single layer on my torso, I'd be pretty cold outside.
Legs just don't care that much.
Changing layers on your legs is more of a hassle, especially with shoes on, and you can't take it to the same level in public without getting arrested.
We do have layers for legs, it's just that most people don't think the extra bother is worth it for most conditions.
Consider long underwear, various types of regular pants, and special wind/rain/snow pants.
outdoors.stackexchange.com/users/1854/Olin Lathrop
I'm not sure why pants don't receive the same attention but the layers are available.
You can easily find base , insulating , and shell layers.
Olin's answer gives some good reasons layering pants may not seem as common.
In reply: (2) There are full-side-zip pants for mountaineers (crampons) and wide-opening pants for skiers/boarders and regular boots.
(3)
Any time it's cold or wet I've used layers, and so did the people around me.
Because a jacket is easier to put on
everyone starts there when that is sufficient of course.
outdoors.stackexchange.com/users/133/Mr.Wizard
Actually, layering of clothing on the other parts of body is discussed, but not so often.
There are many reasons for it: The warming of the chest is crucial.
This is where such vital organs like the heart and liver are placed, so the lowering of the temperature there is the most dangerous.
In fact, your body can reduce the blood flow to arms and legs to help keep the core temperature.
It's easier to layer your chest than your legs.
It's relatively easy to take off your jacket while walking; try to do it with trousers.
Legs consist mainly of muscles, so they are better isolated from cold.
Legs are actually producing a lot of heat while walking as they are doing the most of the work.
outdoors.stackexchange.com/users/557/Danubian Sailor
I live in Edmonton and just walking outside at all means everything needs a couple layers.
Many of my friends routinely wear full snowpants around
, I wear a floor length snow skirt that snaps all the way to the bottom
so it’s easier to deal with than changing in and out of long underwear once at work.
It’s true, legs don’t “sense” cold in the same way
but it doesn’t mean they aren’t cold, and I’m pretty sure my thighs at the knee are frost bitten.
They generally go from cool to sharp pain below -15°C which is a warm day in the winter here, and when we all fill our gas tanks, get groceries and a car wash.
No one is really deterred in their activities even at -40°C.
I have to kind of laugh at windproof cotton/poly pants as being good enough, or that walking makes your legs warm.
Not here, I walk everywhere, and it’s the act of walking that brings the top of my leg in contact with pants and closer to the -25°C outside temp.
My advice is get awesome snow pants with a full zip for easy in/easy out AND long thermal underwear, AND good pants, and based on your activity, use any combo of those.
outdoors.stackexchange.com/users/14333/Jen
There are a few reasons.
This first is that legs are mostly bone and muscle, which if you're walking is doing fairly constant work.
Compared to the head and torso your legs are much less bothered about being a bit too hot or cold.
Secondly it is generally much more hassle to put on and take off leg-wear, especially over boots
so you loose the main point of layering which is to make it easier to adjust clothing to the conditions.
Also if you are walking etc adding layers to the legs can quickly become an impediment which is annoying and wastes energy.
In my experience any sort of moderately windproof trousers (eg polycotton) are fine in most conditions, even when I'm wearing quite a few layers on my body and it is pretty rare that I would bother with waterproof trousers at all.
In colder conditions many people will wear some sort of thermal leggings under trousers indeed before stretchy sports leggings became a thing many people wore women's nylon tights as they add quiet a bit of extra insulation, dry fast and don't add much bulk.
Often the biggest problem is at the waist where you can get a gap between the top and bottom half of the clothing so in really cold conditions single piece suits or salopettes are often the preferred solution.
outdoors.stackexchange.com/users/8820/Chris Johns
Whenever I see clothing articles geared toward hiking and backpacking that discuss layering (base layer for wicking, mid layer for insulation, etc.), they always seem to discuss these things in terms of things you wear on your chest.  How come we don't apply the same principles to our pants?  How come I can't seem to find base layer and mid layer pants when trying to select gear for cold weather hiking?
Why don't we apply the principles of layering when it comes to pants?
outdoors.stackexchange.com
<hiking><backpacking><clothing><pants>
outdoors.stackexchange.com/questions/3312
outdoors.stackexchange.com/users/1912/Ryan Jarvis
Compared to the head or torso, legs generally don't feel cold or temperature extremes in the same way and are often warm from doing a lot of work. It is also easier to layer the chest area, while it is more of a hassle to add and remove layers on the legs. However, layers are available for legs. It's easier to add layers on your chest and legs are mostly muscle, as such they are not as sensitive to cold.
Legs do more work when walking and produce heat. They are mainly muscle and less sensitive to cold. It's easier to put extra layers on your chest, especially when you're wearing boots.