But there's only a certain amount of labor a fixed number of employees can absorb. Imagine a scenario where everyone everywhere agrees to stop returning shopping carts - grocery store employees would be forced to spend their entire shift just corralling them, and then they wouldn't be able to man the cash registers or stock the shelves or whatever else, thus forcing the store to hire another employee on each shift to be the dedicated shopping cart return person.

Logically, every store everywhere tries to run with the minimum number of people possible to keep costs down. The idea is to create a situation where that minimum number of people is increased.