Why are the Egyptian pyramids still standing?

Limestone. Around two and a half million blocks of it.

The pyramids were not built on stone. They were built out of it. The core blocks were cut from limestone quarried beside the site. The smooth outer casing was a finer, whiter limestone, floated down the Nile.

Four and a half thousand years later they are still there. Stone that hard, stacked that carefully, outlasts almost everything built since.

Worth knowing
The Egyptians set those blocks with gypsum mortar. Lime mortar came later, and when it arrived it changed what a building could survive, because it flexes slightly instead of cracking and lets moisture escape from a wall instead of trapping it. The Taj Mahal's marble is set in lime, and it has stood for nearly four hundred years.

Source: UNESCO World Heritage Centre, Memphis and its Necropolis.