A couple reasons I see.
It is hard to get a woman on your level from public approaches and most internet dating. Most guys are willing to lower their standards when it comes to the "easier" methods of meeting women. Women know this, and the younger ones (who are more likely to be open to casual sex) take advantage of this. People tend to meet their equals in situations where there are repeated interactions (school, church, social circles). That's where you have a much better chance of pulling a woman on your level. Also, as you get older, people will try to set you up with women as well.
The other thing is finding your niche. It's difficult for some guys to really find the zone where they can find women that are attracted to their strengths. My strengths were being above average looking, educated, and being up on urban fashion (in the past). Well, on an HBCU campus that did not distinguish me at all. But in a couple small towns where most dudes were losers, I cleaned up. I was fortunate enough to find a niche where I thrived. It's easier for some dude to find their niche than others. Some guys are just in a zone that is not working for them and haven't figured out where they need to place themselves strategically.
The last thing is not related to most guys were are talking about, but it does get a little more difficult with age. It's easier to find someone that is interested, but a lot of women get older and aren't willing to do anything unless it's a relationship (and in this relationship she wants the guy to be marriage oriented). I meet girls that I would have entertained at 21, but now I pass on cause I know she's looking for hubby.