Flawed analogy: Car keys don't grant you ownership rights.
Neither does a private key. The same as a car key, having access to private key might let you access the underlying asset, but it does not mean that the true owner has relinquished their claim on that asset or that you are legally allowed to take that asset. Using the private key or the car key to take the asset which is not yours is theft.
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.
Now that is a flawed analogy. You find this magical document, but you have absolutely no way of knowing that the true owner doesn't also have a similar document and indeed the keys to the car, and is just choosing to use public transport for a while, or is working from home, or has decided to walk/cycle everywhere to stay fit, or is keeping the car as an investment, or is waiting for their child to turn the legal driving age to use the car, or 100 other reasons that they might not be actively using the car, and then you swoop in unannounced and take it from them against their will.
Your inability to track down and contact the true owner
especially when that owner may very well still have full access to that asset and simply be choosing not to use it for the time being does not make it OK to deprive the owner of that asset.