The writing team for longer seasons needs some guys with skillsets like Don Bellisario, Glen Larson, and Aaron Spelling. I mean, look at old shows like The Love Boat and Magnum P.I. How many different ways can you tell the same story over and over, but those shows were hugely successful and ran for almost a decade nevertheless? And I don't buy any tripe about modern audiences being more sophisticated. On the contrary, they suffer from incredibly short attention spans and a lack of critical thinking skills. Everything has to be spelled out for audiences who can't stop scrolling their phones while they 'watch' a TV show.
I kind of got onto a GenX rant there, so back to the original question: why not longer seasons?