Makes sense, the king dies unexpectedly, you need a tomb real quick. It probably takes years to dig these tombs and that's assuming you've found a suitable place to dig. You pick an existing tomb of a less popular ruler, wall of some sections (but maybe leave the artifacts?), renovate the new main chamber for him, and put a new name out front.

It's like those dilapidated buildings in NYC where the ground floor is some store but everything upstairs is basically uninhabitable decay. The store makes enough rental income to justify not renovating the upper floors into something actually usable.