That is a possibility but I would suspect one may want to leave their car at the resort if staying at an MK resort and going to MK that day, staying at an EPCOT resort and going to EPCOT or possibly Studios, or staying anywhere other than an MK resort and going to MK because the bus or boat trip from resort to MK can potentially take less time than driving and parking in the MK lot which requires parking > tram to TTC > boat or monorail to MK.
This. I actually prefer renting a car and keeping it at the resort. Parking is free at the resort and theme parks, I get to skip the LOOOOOOOOOOONG DME line at MCO (it was fun to do a couple of times, but the wait at the airport has grown so long that I'd rather just haul my own bags), I don't need to wait 45 minutes for a bus at 11:00 at night to get back to the resort, and I can easily take a run elsewhere in Orlando (say International Drive for dinner) or even out to the Cape if I decide to. We just leave it at the TTC or Epcot lot and take either the Monorail or Skyliner if we park hop. Surprisingly, I think it's actually proven to be most valuable going to DS for dinner. Talk about long waits for busses (or boats).
I've found that 2 or three trips on Uber and I've paid for 1/2 my rental car. If I use free rental days, it becomes a wash, cost-wise.