Just guessing, but it's probably due to the physical changes in appearance and the fact that they're on their last leg as far as trying to have a baby (if they don't have one already). It's probably pretty difficult to all of a sudden start getting treated differently because you're not that bad anymore. No more people smiling at you, no more offers to buy you drinks, catching dudes staring at the young chicks and paying you no mind. It's humbling. That's probably why so many turn into cougars and start going for younger men. It's a fantasy for a younger guy to get with an older woman but for a dude their own age they just seem used up and past their prime.