Same reason why a lot of guys naively believe that women become desperate and unwanted as they get older...wishful thinking. (See: "The wall" myth)
While the majority of guys are not straight up truecel....the majority of guys are not happy with their obvious lack of SMV. Most guys have to take what they can get (which in many cases is a sub3 woman and/or a single mother), deal with a lot of rejection, get dumped, get cucked, beta bux, go through long dry spells, etc. They're just obviously not going to be vocal about it as they don't want to come off as "bitter losers" or whatever.
Some simply accept their lot in life...but others desperately want to believe that when they get older, things will get better for them. They want to believe that if they work really hard when they're young, climb the corporate ladder, get that promotion and that nice car by the time they're 40, the shoe will be on the other foot and 40 year old women will be desperate and throwing themselves at them. Of course in reality, even if they do become financially successful, if they find themselves still single by 40, the best they can hope for would be to betabux for a single mother who won't be attracted to him and will cuck him for a much younger guy, and divorce rape him.
It's too bitter of a pill for most young guys to swallow that this IS their peak and that's it's only downhill from here. It's an even more bitter pill for a 25 year old to swallow that he is already past his peak (in terms of how attractive he is to women).
Take the agepill.