Because too many men give up courting their wife once they "have" them. They make sex boring, rote, and entitled - all things that are a turn off to women. They don't do their share at home, leaving women exhausted and distracted by having to do all of the emotional labor and most of the physical labor as well. Women want exciting, connected sex. They want to be married to a real partner. They don't want to have to service a man, even a man that they love, out of obligation.
Read this phenomenal story from a woman in her sixties who finally discovered what actual passion looks and feels like for a much more in depth understanding of this dynamic.