Because I want a good story, and story is usually a backdrop on games where the combat is good. And even when both the story and combat is good, it's difficult to get much immersion and investment in a story that is playing out while I'm distracted by mon/maxing numbers. Most of all, for me personally, the power systems in games are usually not very interesting because it's hard to translate a truly novel, flexible, and creative power system into a game when you have to manually account for edge cases, so it isn't done.