I do get where you're coming from, though. Why is it not your keys, not your coins with cars, too?
The main (and only) difference is: You don't get the ownership rights with the car keys. Just because you can drive the car, it doesn't mean it's yours. Bitcoin private keys, though, do grant you those rights. Whoever owns the keys, owns the money. It's not how I see it, it's the way the protocol sees it.
Someone generated that private key. That's the sole person who should have ownership.
Forget about ethics for a while. Who
should have ownership versus who
does have the ownership differs. The protocol listens to the latter. And since ownership is defined by the protocol, whoever has the keys is the owner.