Did you know that in the Netherlands, graves are routinely cleared after a couple of decades? Graves cannot be bought, only rented. The usual period is 20-30 years. After that term is up, the grave is cleared if nobody renews the lease.
Most cemeteries have a communal plot where any remains are reburied. In most cemeteries, it is unusual to find graves older than 50-100 years. An exception are Jewish cemeteries, as Jewish tradition prevents them from being cleared.
See also this overview of online cemeteries.