The key value is in avoidance of loss (burglary), not in needing to get a new key cut.
Losing a key means changing the locks, the more duplicate keys the higher the cost.
Except that getting the key back still means changing the locks because the finder could have duplicated it, so it seems like there's not a lot of value in giving it back except in the case that it's their only copy that they will use until they change the lock.
I'm quite impressed how in most countries, it's a function of empathy vs. how much hassle it is to return things.