Because the concept of "ownership" doesn't really mean anything unless it can be enforced. What's the point of owning a house if you can't live in it because someone else is squatting in it?

Since there doesn't seem to be a way to decentralize the enforcement of real property ownership (as enforcement could involve violence, which the state has a monopoly on given our current social contracts), we're stuck with centralization in at least one component of the "ownership-enforcement" pair...

So then what's the point of decentralization at all?