Usually land is some kind of rectangles

Makes sense. You’d need to do half-circles on the edges to tile rectangular land in hexagons, and that would mean more machines and pipes.

Hexagons would only make sense if you needed multiple, equal-sized irrigators, but the more I think about it, there probably isn’t a size limit on these machines. They are built in modular sections, with each section driving itself relative to its neighbors. If they need a larger area covered, they’d build a longer irrigator.