One problem is that there's a long tail of car use cases that are served terribly by things like car-sharing. For example if you rent a car for a few hours of driving, two weeks of parking at some trailhead or remote resort and then driving back it's inevitably priced for the wear and tear that would happen if you spent the entire two weeks doing nothing but sleeping and driving. Rentals that aren't dead-set on the pattern "a few days of driving between arrival and departure" exist, but they have terrible discoverability and that implies that prices fell arbitrary ("is this close to the price I would get in a market with full visibility or are they just taking advantage of nonstandard needs?")

I guess what I'm trying to say is that there's still plenty of room for innovation in the non-owned car market.