In the cases I was familiar with, they had problems with their fathers, and often broken families.
I've mentioned this before. When I was 21 and in college, a friend bought a girl to my house. He fell asleep on the couch and she immediately hit on me. I declined, since my friend was there.
Later, he mentioned she had told him her father had taken her on "fishing trips" that apparently included handjobs. And she was promiscuous as could be.
Before that, when I was 18 and sharing a three-bedroom apartment, a 19-year-old divorced girl came over and though the night went from room to room. Divorced at 19? I sometimes wonder about her family life.
I've also had three naked women jump in my bed. I know for a fact all of them had family problems, especially with their fathers. The last one turned out to be a drug addict and part-time blowjob whore. She told me the last time she saw her father he opened the door, threw some money at her and slammed the door shut.
Those are extreme examples, of course, but sometimes those extreme examples are indicative of widespread societal problems. I've mentioned before the word "monster" means "demonstration."
A "monster" is a demonstration and a warning of problems to come.
The same applies to PUAs. The ones I've known had problems with their families. Big problems. And all of them had drug problems.
So I conclude the more families fail to form, or break up, the more sluts and PUAs we are going to have.