I'm not talking about "tunnel tombs." I'm talking about the mortuary chapel probably used during the Sed festival in life and where offerings were made after death. In earlier royal mastaba burials, there was a unsealed portion of the tomb used for these purposes. With pyramid burials, the mortuary temple outside the pyramid served the same purposes as the unsealed portions of the mastabas.

A simpler explanation for why the Pyramids of Giza were not usurped: By the time royals began routinely usurping earlier royal tombs (late New Kingdom/early Third Intermediate Period), the pyramids and mastabas of the Old Kingdom were long out of fashion as burial sites.