Besides what bcrosby says, old people hold on to real estate "as an investment". In many cases they actively prefer that it be unoccupied, because occupants complain and want repairs done, and so on.

Or, in the case of "world cities" like London, wealthy (nearly all old) people from shaky and shady regimes around the world buy them as places to escape to if need be.

As well as falling household size in occupied dwellings, the proportion of unoccupied dwellings in these cities increases far above the percentage normal for the delays inherent in ordinary buying and selling.

IIRC Paris is attempting to combat this with a surtax on dwellings unoccupied for long periods.