A guest with a car is more likely to drive off reservation and spend money elsewhere. Disney wants a captive audience.
side story circa 1980s. Christmas Break Disney World FLSA Disney employees fired today for parking prank. 5 disney parking employees were dismissed today for parking all red rental cars in one area. Puzzled guests took up to 2 hours to find vehicles after spending all day in the park and became impatient as their children cried from exhaustion. Disney would not release the employee names saying personnel matters are confidential.
That's the key. I haven't been in years, but when we did, we had a car so we could do other things other places, including buying groceries...both during our Disney stay, and afterwards when transferring to a different part of the vacation. We always took a day or so to travel to Canaveral, or spent a day travelling around the Orlando area.