IIRC, their society depended on it. During peacetime, it was a major economic engine paid for from the royal treasury, with the work starting as soon as a new pharaoh ascended the throne, and the labor provided steady work when agricultural activity stopped due to Nile flooding. Of course, it's only in hindsight with our modern understanding of economics that we can see how such an activity at face value not important to any basic survival need was in fact a key piece of the economic fabric.

> The Roman writer Pliny the Elder, writing in the first century AD, argued that the Great Pyramid had been raised, either "to prevent the lower classes from remaining unoccupied", or as a measure to prevent the pharaoh's riches from falling into the hands of his rivals or successors.

This was the Pliny view.

Aka, it was a jobs program.

If only our current leaders could have the vision to build robust infrastructure