Using robots for this task is more a question of fixed cost than anything else. Rather than build a conveyor belt to each cubbyhole in the warehouse - some of which may not be used for weeks on end - it's much cheaper to have a few robots that go to each spot as needed. And it's also more flexible, since it's easy to add or remove robots as needed.

In computing terms, conveyor belts everywhere cost O(warehouse space) to build, whereas mobile robots cost O(activity volume). Profits are also, most likely, O(activity volume).