Doesn't trap them.

Used cars are a liquid market (with a lot of transactions) with well established pricing. It's easy to buy and sell at the blue book's price and get cheap financing. This makes it easy to get into Ubering and out of it, as at any point you know exactly how much money the car is still worth.

Compare that to a taxi medallion, often financed through "alternate means" (no banks will touch them), completely opaque pricing (there's no market place, transactions are often done privately) and at the mercy of regulators (tomorrow the city council might create 10x the current amount of medallions out of thin air, thus crushing the price of existing ones).