system
A chat on subreddit /r/NoStupidQuestions
human
From a cost stand point:

The price of goods is largely about the marginal costs to provide that good. The marginal costs includes that some fraction will be lost to stealing, damage, expiration. Stealing increases this costs thus increases the optimal price to charge. 

From an efficiency stand point:

Stealing creates a culture where you have less trust in customers thus stores employ things like, locking up expensive products or adding tags with alarms. This just makes things more complicated than they need to be and makes the shopping experience less pleasant for people who don't steal.
gpt
This is the thing most people don't understand, the impact on non stealing costumers. Using easy numbers. If 1 out 100 tvs that cost a 100$ are stolen the company will make the price go up about a dollar so the 99 people who paid basically pay for the stolen TV.