The vast majority do return carts in my country. But when you have 100 customers an hour (which isn't much) and 5% don't return carts that is 5 carts per hour - odds are you will always see a couple carts out of place. The numbers above are made up of course, but the point remains that a small minority makes everyone look bad.
Only one store near me has a coin deposit: Aldi which has European roots - it has probably never occurred to them to check if the cost of the coin collectors is worth it (I'm sure it adds $5 to the cost of a $300 cart). Every other store finds people return their carts often enough that it isn't worth the bother to put those in place.
And what country is that? If you're comfortable sharing.
Of course the coin thing somewhat relies on coins of mentionable value being in common circulation, which rules out the US from the start.
Here few people still carry coins since contactless, but most still have a fake euro coin somewhere on their keyring or in their car for shopping carts.
They hand out these fake coins, which are usually branded with the supermarket's logo, for free inside the shop. But the system still works because having to go inside for a new coin negates the saved effort of not bringing back the cart. And there's probably some feeling of discomfort associated with abandoning the cart with your coin still in it.
The US $.25 coin is similar in size to the .50 euro coin (about half the value). It is in common circulation and most people still have them. Most people I know keep a stack of them in their car for use in the carts.
Only one store near me has a coin deposit: Aldi which has European roots - it has probably never occurred to them to check if the cost of the coin collectors is worth it (I'm sure it adds $5 to the cost of a $300 cart). Every other store finds people return their carts often enough that it isn't worth the bother to put those in place.