> You're talking of the Giza ones I assume, which were indeed surprisingly text-free. Might have been a 5th dynasty "invention"?

Exactly. Think of the purpose the inscriptions inside a tomb. Nobody is going to see them, so why bother? The Pyramid Texts mark the beginning of an Egyptian magical tradition where writings and images can help the deceased in the afterlife. This tradition apparently started with Unas in the 5th dynasty and developed into the Coffin Texts, Book of the Dead scrolls, and finally hypocephali at the very end of Pharaonic Egypt.

With Khufu (4th Dynasty, Great Pyramid), the inscriptions are in his mortuary temple right outside the pyramid because that's where people would actually visit, not the interior of the sealed pyramid.

Still, the proscription against tagging pyramids held, somehow.

The Valley of Kings tunnel tombs are lousy with texts, sealed in. So it can't have been meant for visitors.

I am always mystified at how there could be never any soot on the ceilings. We look at these things all brightly lit up, but people writing them were in a very deep hole in the ground.

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.

Just a thought, but isn't it possible they were written while they were still being built, so they would be open to their surroundings and well-lit with natural light?

That could happen with the pyramids, but almost all of those are blank inside.

The Valley of Kings tombs were tunneled deep into living bedrock. How they did, I do not know, but ancient people worldwide seemed to have no difficulty cutting unlimited amounts of stone.