It's not weird in the slightest. Mummified bodies in Egypt and elsewhere have been stolen out of their tombs for millenia. Because people would pay a lot of money for them. That's what's believed to have happened here. The British weren't the first people to do that, it predates the existence of the UK. But some of the sarcophagi that were left behind were too big to steal, they couldn't get them out of the pyramids. Which indicates that they were there first, long before the roof was finished, which seems to point to the fact they were important and necessary to be inside the pyramid, maybe the main reason for the pyramid's existence in the first place.

The reason there were little to no hieroglyphs on the walls is because that's a practice that didn't start until the next age centuries later.

But as others have said, the pyramid's of giza are basically a big graveyard, there's tons and tons and tons of tombs and mastabas surrounding them, which were used to bury people. And those weren't the tombs of lowly workers who died making it, you had a to have a good deal of money at the time to have one, especially on a site that was so important.