During the fertile season, right after the floodplains were flooded, you needed an enormous amount of labor to take advantage of the huge area of freshly-fertil

During the fertile season, right after the floodplains were flooded, you needed an enormous amount of labor to take advantage of the huge area of freshly-fertilized soil. Presumably this area needed to be tended and maintained, and then of course the harvest was another time when you needed enormous amounts of labor. And then you've harvested and the plains are briefly fallow, then they get flooded by the Nile for a few weeks, and there's nothing to do. So you have an enormous amount of labor power with nothing to do for a couple months every year. Why not pay them a little to lug massive stones around to build a monument to your awesomeness? Better than having them idling in the streets!