I don't think being a single mother is about preferring it that way. If your talking about women who got the child when single, and not a mother with kids after divorce, they often desire a child so much but they cannot find the right partner to have them with so they decide to do it alone.
A tough choice, especially since you cannot fathom what it's like to have and raise children until you have 'em.

There's also women who get pregnant and the guy does a runner, leaving the mother pregnant and having to decide whether to keep the child or not.
I know one, my daughter's age, who had that happen to her. She kept the child but it's not easy. Financially struggling, also struggling with time, and there's no one to help you out like when you have the father there too. In this case the father does nothing, doesn't pay anything etc. etc.
She said the worst thing is you have no one to share the good and funny stories with. Sure there's grandparents but they don't know and see the child the way a parent does.
So even in that sense it's a lonely road and she'd much preferred it differently: with the father of her child.

I myself have been a single mother after divorce. And hell, that was NOT what I preferred at all!
My ex did help out financially and had the kids every other weekend.

And selfish? Excusez moi!? I made the decision in my children's interest, especially my son's.
And even if there's no such reason, if parents do not love each other anymore the way they should it's much healthier for the kids if they split up. Living in a home with a very ill vibe, parents maybe arguing and simply not being happy is one of the worst things you can do to your children.
In that sense deciding to divorce is the best thing you can do for them. it brings back peace and happiness in the home, of course after a period grieving and of getting used to the new situation.
But children adept quickly. Not, however, to living in a stressful situation with 2 parents who come to dislike or hate one another.