There are tons of valid reasons why many interesting and pretty girls are single.
Lack of valid suitors
Lack of time
No desire to commit
Not actively seeking a relationship
Not putting themselves in situations where they meet tons of guys
The same reasons a lot of PUAs are single. The same reasons a lot of naturals are single.
My current girlfriend(soon to be ex) was single when I approached her. HB8, athletic, reads a lot, brutally honest and very blunt in general, doesn't put up with BS, hard working and very ambitious. Works full time and goes to university while still finding time to work out and play flag football. She doesn't do the whole night game thing, she barely drinks.
I picked her up at work(bookstore) and she told me I was the first guy to have approached her so directly in her whole life. I am pretty sure there are tons of girls in similar situations due to the amount of similar reactions I've gotten during day game.
This brings me to another point. Majority of guys are AFCs. Why would you assume most girls are taken when most of the guys out there are AFCs? Illogical
So yeah all that to say, some people have other priorities in life that take precedence over actively seeking a relationship. Being single in no way indicates something wrong with you.
I'd be more inclined to say the validation seeking girls who have boyfriends but are in the club for the sole purpose of getting hit on by tons of guys to make themselves feel better are the ones with some very deep seated issues, but I digress.
Sorry for the rant