Flawed analogy: Car keys don't grant you ownership rights.

But: If I found some sort of magical document which whoever found it, gained legal ownership of the car, that could have come to my hands by either mistake or deliberately, knew no owner's name, be unfortunate at succeeding to find contact with them or some family member after a year of trying and had no obligation to give it to any other person or state, I'd consider it mine.

It might sound ridiculous to happen in real life, but not in the Bitcoin network.