Some of the earlier pyramids did crumble. They made mistakes and learned from them and innovated over time. The pyramids aren't still standing (just) because of the materials, there's real structural engineering at work.
And later pyramids. As a matter of economy, many were constructed from mudbrick and only encased in true stone. Over time, particularly after the casing stones were removed for other projects, they collapsed into the rubble piles referred to as ruined pyramids.
