I'm quite sure a lot of the pyramids and tombs are very unsafe. In addition to the risk of additional damage due to thoughtless people, graffiti, vandalism, theft, etc., the tourists themselves would need to be protected in so many of them. The tomb of Ramesses II has been virtually destroyed over the years by flooding, causing the rock to swell and crack and then fall in on the floors in many parts, like the main burial chamber which is in terrible shape, very little remains of what must have been a spectacular sight (what a shame!). But from what I understand the ceilings are unstable in a lot of it still.
Can't have tourists getting crushed by large slabs of loose masonry, you know?
If you look at the plans for the substructure of the Step Pyramid and extrapolate a bit, it becomes apparent that there are a lot of places that the unsuspecting tourist could get into some trouble. We all like to think we would be respectful and cautious, but so many tourists are not!
I agree about wanting to explore this most fascinating of monuments. There are so many passages and chambers, much more involved than in most of the rest of the pyramids--it would be very exciting to wander around under there!