The average person in the dating pool is bad because the good ones get into relationships quickly and stay in them. (Edit: This statement is a bit hyperbolic)

When people complain about the types of people they date or might have dated, they are complaining about the group of people mostly composed of those who have, for whatever reason, failed to get into a long-term, committed, stable relationship. Those reasons often aren't very good, and the effect gets worse over time as the best prospects shack up.

As you close in and eventually surpass the average age of marriage, the mere fact of being single is a red flag. You'll start to encounter more and more people who are some combination of already-divorced, single parents, have major weight problems, very high disagreeability, have extremely high expectations, polyamorous, addicted to high-energy but unstable lifestyles, severely mentally ill, very passive in dating, just downright not attractive, or some other problem.

The stuff that RPers complain about is present at all ages partly because the women who will fail to get into LTRs are present at all ages. All women are hypergamous and other things RPers proclaim, but there's a lot more variation among women than they give women credit for. It's those hyper-hypergamous women who are going to have a seriously rough time finding and staying in a relationship they can feel satisfied in.

Edit: All I'm really saying is that the people who are the worst for commited LTRs have an outsized impact on our perceptions of the opposite sex.