Most single women are very normal people - they don't need to change. There isn't anything 'wrong' with them, apart from society making them feel like lepers or to be pitied in some way.
Hear, hear @LuluJakey1.
The vast majority of people who aren't in relationships are exactly the same as people who are in relationships -- they are no more or less attractive, troubled, pleasant, opinionated, demanding, nice to be around etc.
To turn the question around (as I think someone suggested upthread), it's interesting to think about why friends who are in longterm relationships/married are in those relationships, and not single.
Several of my university friends did teacher training, went straight back to their home towns, got a job in their own old school or somewhere close by, and married their childhood sweetheart/local boyfriend -- it fitted their life plan, and their boyfriends', which was to essentially replicate their parents' lives in the same place. And it seems to have worked.
My closest male friend an introvert with a horror of the unfamiliar definitely wouldn't be married with children (by his own account) if his longterm, long-distance girlfriend hadn't got fed up, proposed, organised the wedding, and pursued IVF when they didn't conceive naturally. (In fact this marriage is likely to end soon, because, unfortunately, he's completely unsuited to the daily demands of family life... I imagine he will remain single by choice then.)
Another very good friend (who is bi) had a serious relationship with a man on a postgraduate year abroad, then had a nervous breakdown after she'd returned to her own country, in part because of her highly-pressured PhD course, dropped out and went to live in his country, where she's now married to him with children -- happily, but not without regrets for the path she left behind. If she'd been happier in her studies and hadn't had a famously awful supervisor, she would (again, by her own account) be likely to have pursued relationships with women in her own country.
Another friend is married to a man she used to laugh about when casually dating him because she thought he was a bit of a nerd -- but then she had a serious car accident, he was enormously caring and nursed her back to health, and they married and have children. If she hadn't had the accident, their relationship is unlikely to have made it past casual dates.
The reasons people are married/coupled up are as various (and, I think) as essentially meaningless, as why people are single.