I think there are a lot of reasons why someone might have children with an clearly abusive, lazy or otherwise unsuitable man.
I think the age of the woman involved, her self esteem level, culture, embarrassment and upbringing have a lot to do with it.
I have noticed quite a few times the man can be quite a bit older and/or she was relatively young when the relationship started. I think sometimes dodgy men look for younger and/or less confident women who will have a harder time dealing with mistreatment, then the women get accustomed to it over time. It's day to day life eventually, finances get entwined, etc. It starts to feel normal and he will tell her it is, and it's harder to leave or start over each day it has continued.
If she's had a difficult relationship with her parents, or grown up in a dysfunctional home and lacks social support that can massively contribute. She might not know what a normal relationship looks like even if it feels wrong, know no one will help her leave and so tries to follow the expected relationship steps. She might even be under pressure to stay from family, who will say things about how serious or bad it would be to leave. He might seem great or acceptable to others.
It might not be till she's older and/or grown in awareness and self confidence, a mother or soon to be, that she starts to feel it's really not okay. It never was of course, but she might not have been able to acknowledge it before.
She might be afraid of him and to deal with the fear has convinced herself it's HER choice to stay, and married, had kids etc due to this denial. Also, perhaps the partner's pressuring.
She might also think she can rescue him from his issues or that if she loves him enough he will change, or that a baby might somehow fix him and the relationship. -- This might be very naive, but some media does portray this message. That women can save difficult men.
It very sad it comes up a lot here, but I think there are usually reasons which are logical from the perspective of the women involved but harder to grasp if you have not got the same history, personality or experiences. -- I think such women need support not judgement and blame.