It's more convenient to query a database than to ask everyone.
This requires some social contract to know who the authority is, and that's probably based on how much they have spent on marketing to become The Authority.
It always comes back to social contracts.
if there are 20 NFT chains, and 10 of them assign ownership to A, and 5 of them assign it to B and the other 5 don't assign it at all - what can we say anymore about the ownership?
all of this public ledger stuff might make more sense if, unlike as is customary, there is a single authoritative chain instead of a 'marketplace' of chains.