They can track their cars just fine, but they can't project the movements of specific vehicles into the future; sometimes people extend their rentals or want to return them to a different location, sometimes a car has a problem and has to go to the shop, sometimes (often) people are no-shows so they rent the car to somebody else. In order to have a system that doesn't fall over any time any of these things happens, they don't attempt to make promises about any particular car being on any particular lot at any particular time.