You can still sell the parts years and years later. No garage necessary. No driver's license required. If someone gives you a car unless it's a total junk you can easily convert it to money, and the time pressure is completely different compared to a pet.

The moment you accept the pet you have to think about everything in your future for years. You accept a car? Okay, you might not even see it, maybe you just get a paper and a key fob and an address. And you don't have to do anything for years. Nothing happens morally.