[jhbadger]

We absolutely know they were tombs. We can read Ancient Egyptian you know. There are texts on the walls dealing with how the person entombed there will deal with being judged in the afterlife. The idea that that the pyramids were mysterious in purpose only made sense prior to the mid 19th century before we could read the hieroglyphs.

[moloch-hai]

There are, in fact, no such inscriptions in any of the Giza pyramids, and no remnant of funerary apparatus was found in any of them, never mind a mummy. (Other looted Egyptian crypts were found full of trash; looters don't clean up after themselves.) The only writing found inside was not funerary.

There are funerary temples lousy with inscriptions outside, but anybody can build those anytime. Crypts elsewhere are scribbled throughout.

We still know very little about the pyramids, so cannot contradict speculation. The official story / timeline is just one such.

[jhbadger]

The Giza pyramids are far from the only Egyptian pyramids though -- the reason the earliest descriptions of the Egyptian religion are referred to as the "Pyramid texts" is that they are found in other pyramids, some of which like the Pyramid of Unas are nearly as old as the Great Pyramids in Giza (but better preserved as so more informative from an archeological perspective). The Giza pyramids are impressively large but seem to have been entered/looted at various times in antiquity.

[moloch-hai]

They certainly would have been looted, if there were something to loot. But as noted, looters never cart out the trash. (Or, anyway, until recently, in order to feel more like legitimate archaeologists.)

Anybody can co-opt a spare pyramid and scribble it up. There is even an absurdly sparely applied term: usurpation. Usurping one of the Giza constructions would raise eyebrows, but the others could tempt, if in good shape.

Once one's immediate predecessor is securely dead, actually expending on a fancy interment is not strictly necessary, given nobody can see inside anyway. Lots of crypts were left comically incomplete, later. Even Menkaure's pyramid was not clad as specified.

Later they must have got the idea of having supporters demand to tour the crypt before it was sealed, to make sure things were actually done. You probably can determine which successions were ... unfriendly by how incomplete the funeral chores are.