A data point: speaking as someone who would require major surgery to even be considered "average", I think it's indeed an effect that comes from many different factors all pointing one into the same direction (I would also assume there exist women with similar experiences, although not as many as men).
The "why bother" effect became very strong in me after my last long-term relationship failed, and I noticed that my advancing age made it even more difficult to get anyone interested in starting a new thing.
My personal disillusionment reached its peak when I realized that dating and marriage are one of these treadmills that just eats away at your life. People who are physically attractive, well-educated, and wealthy are in my opinion much less likely to feel this drain - both because they have more attractive options to choose from and because they tend to have a more gratifying life in general. Whereas people like me would have to expend a considerable amount of energy just to even get an opportunity, only to end up with a less-than-ideal match (because if I'm "settling" for someone, odds are she's also somehow forced to "settle" with a less-than-ideal partner such as me), and I'd have to spend an inordinate amount of effort just to keep up in the long run. If you're someone who is constantly getting the feedback that you're inadequate, every step of this is degrading. So why do it?
So the realization is that striving for partnership is not worth it for many men (and I suspect for some women, too). It's perfectly possible to lead a fulfilling life without it, once you stop running after the proverbial carrot that society would like to dangle in front of you. Instead of being someone whose primary attribute is being a sub-par partner, I can concentrate on being actually valuable at other things that matter to me.