ELI5: Why don't we already know everything that's inside the pyramids?

Two people have answered that the pyramids are “incredibly complex”- they aren’t- they’re giant piles of rocks, with narrow shafts and a few small rooms inside. We know about those because the shafts reach the surface. There may be other rooms, but it would require tunneling through tons of rock to access them, because there are no doors or hallways to these rooms, and millions of tons of rock on top of them.

Explain Like I`m Five: good questions, best answers.