Be it casual sex or a relationship, people are willing to compromise on looks to achieve their main goal.

When someone's goal is sex and they'd rather have subpar sex than no sex, they're not going to be picky with their sex partners.

When someone's goal is a relationship, compatibility and stability are the most important aspects. So unless the person's hot, they'll have to prioritize a good stable partner over a hot partner.

"So women do practice AF/BB!" So do relationship-minded men. I know plenty of guys who'd fuck a 10/10 bimbo but would rather marry a wholesome 6. Also, nymphomaniacs exist.

That's why we have certain people who are picky about casual sex and relationships: It's not what they want and they have to find an exceptional person to be that exception. Otherwise, they'll forego casual sex or a relationship until a more appealing person comes by.

"But what if they're not willing to lower their standards to get what they want?" They're either hot or they're picky beggars. It's reasonable that a 9 isn't going to settle for a 4. Now, a 5 needs to get over themselves or increase their rating (perhaps not just in the looks department).