The logical answer is she just hasn’t met someone who has that gut instinct that he should be with her. I say that because we all know men in relationships with women who are crazy, rude, have baggage, etc.
BUT here are some other reasons:
She likes white boys, and they don’t want her
She like black boys, and they don’t like her body
Her teeth could be messed up
She could have kids, and multiple baby daddies
She could have a reputation for being easy, since she’s most likely let a number of those men smash, but they never committed
She let them smash in the talking stage, and the sex wasn’t good enough to commit to, but they may have kept her around to use as a human jerk machine
The men are intimidated, and don’t think they can please her
She is very demanding and needy too soon, and scaring them away
She has a lot of childhood trauma, and is revealing those things too soon, and that can be overwhelming when you’re dealing with a woman who seems like she needs therapy, and will depend on you to be her only source of happiness
She has bad hygiene, a messy car or living space
She’s trans
She can’t hold a conversation
She’s telling the men that men never like her, and they think “Maybe I shouldn’t like you either.”
She criticizes the men too much
She doesn’t leave the house, therefor doesn’t meet that many men in the first place, so despite getting rejected, her numbers are low for men she likes
She gets rejected by men she wants, and gets attention from men she doesn’t want (ugly guys)
She’s a bytch. But again, we all know someone dating or married to a bytch, and wonder why the hell is he with her.