Because our society is built around couples and families. That is the standard model. It is actually very hard, socially, financially etc to be single long-term and not conform. You are the exception to the rule.
(If I became a single mother, if be in the vast vast minority in my kids school for example.
I also suspect some mums attitude would change towards me, because the you're a loose cabin and a threat. They think you'll be looking for a man and perhaps willing to take one who's not single). Single women are treated like this by other women all the time .... Lots of posters can say they'd be er do that but sorry, ime, you are treated differently and sometimes excluded.
Because they want kids, need men to have kids and then feel they need them to afford the kids.
Because common standards for men are extremely low, for a number of reasons. When an acquaintance's sister left her husband for going up his job to start a business and not starting one, playing games all day instead while she paid for the household imc the child care, and then he refused to drive her to the hospital during a miscarriage (the breaking point) she was told by numerous people that if he wasn't cheating on her or beating her, she shouldn't be divorcing him - she was being ott in divorcing him, esp because she has a son with him.
When my partner was physically threatening to me ..my sister said "they're all like that, Xenoi".
Back on the first example - people believe that families should be kept together if there are kids involved + and this heavily influences women.
On the low standards; many men are shit and our culture (most cultures enables them being shit) and until recently they had massive power over women and kids, so their suit behaviour was gotten away with/standard. And we still have that. It's not so long ago that women were referred to by their husbands first name as well as second, when they wouldn't have gotten a good deal in a divorce court, when they could be raped within marriage, when the police wouldn't take violence against them seriously.. it was just a "domestic" that she must have contributed too by being a nag etc. I grew up surrounded by things catering to men and their sexuality.. page 3 in newspapers, topless calendars, the Benny hill show and carry on laughing, porn mags that got found etc
We are not far down the road from all that. We are barely a second, in teens of the long-term, on from that total dominance and forced acceptance, and loads of people still think that way.
They think all men are pretty much shit, so they're not going to get better....and you can't be on your own because that's failure.