Many men are very well aware of the fact that they aren't very good in bed. Why they don't bother to do something about it I cannot really explain very well. Lack of an ability to express empathy is probably part of it...
It's all kind of intertwined into the details surrounding patriarchy, using one's penis as a measure of power, and the fact that men are just generally very insecure creatures to begin with while socially they're not allowed to actually show this. Thus, when it does peek through, it's because there is a massive amount of suppressed insecurity at that point.