"the vast majority of rational sane women would know not to have a baby with an abuser."
This is dangerous magical thinking.
Obviously, obviously, if you were picking potential fathers from a line up of men you'd not pick the one with angry eyebrows and fists clenched with a t-shirt that said "Where is my sammich?" on it. That's overly simplified of course but it's extremely easy to look at somebody's relationship or a man you know from the outside and think "God, who on earth would choose to have kids with him?"
That's the false sense of security, though.
Abusers do not start out abusive. Some, especially if they have narcissistic/controlling personalities (and all abuse stems from control, at its root) can come across as extremely charming and pleasant. They can often tick "all the right boxes" for fatherhood - stable job, well respected, authoritative, intelligent, etc etc. But if their abuse doesn't begin until pregnancy or after birth (which is extremely common as a pattern) then these things also count against you because you realise that any accusation of abuse is going to be difficult to believe as most people see this very clean nosed persona.
Or sometimes men who later turn out to be abusive hit a different kind of right box in women who want to be mothers - another common profile is the "little boy lost" or loveable rogue with a dark past, who struggles with his demons. This can start out fairly benignly and before real children actually come across some women can feel very strongly nurturing towards this type and almost "adopt" him in a kind of mothering way, feeling that love can fix him or that they aren't perfect themselves so you have to forgive somebody their imperfections. You can be very rational by the way and still believe this because it can also be an assumption that although he has his troubles, he would not take them out on a person he is loyal to aka you. Unfortunately what tends to happen is that little boy lost can't handle no longer being the centre of attention and throws tantrums, which are harder to handle in a fully grown man who is bigger than you than in a two year old you can scoop up under one arm.
There is another common story where the abuser "rescues" a woman who is in an emotionally vulnerable state and giving her a "real family" is a part of that whole script, although he then gets angry when she is not pathetically grateful at every single thing. I assume that this isn't counted in your "sane, rational" statement though as somebody who is emotionally vulnerable might not be the most rational, but these rescuer-type men can be extremely convincing and appear very supportive and as though they are encouraging independence, until suddenly she starts to stand up to and be independent in relation to him as well and he doesn't like that.
In short it's not like women commonly choose to have children with abusers - they choose to have children with men they believe will make good fathers, and the abuse starts later. Sometimes it doesn't happen like this, and the abuse is already clear by the time children come along but remember that abuse being clear from outside a relationship is miles away from it being clear from inside the relationship and such men are also extremely skilled persuasionists. It actually suits them down to the ground to have their women knocked up as soon as possible, partly because it ties her to him, partly because it is a power thing, proving his masculinity, partly because everyone will congratulate him for it when he actually has to do very little work. Partly because it gives him the most valuable emotional crowbar. It works to prevent her from leaving (Guilt: The children will lose their father), to continue to abuse her after she leaves via contact with the children, and to make her look "crazy" or unreasonable to others (she never lets me see the children!).
The other point is that most first time mothers have absolutely no idea what having a child is really like. Whether they are with an abuser or not. You can't adequately explain or communicate how motherhood actually feels, affects you, changes you, and this is of course in part because it's massively different for everyone, but it's also because it's just such a huge inexplicable thing.
People think that a baby will smooth out problems in their relationship, but it actually forces ice water into those cracks and blows them apart. The relationship has to be strong in the first place and you have to both be invested in holding on across that gap for it to stay strong. People think that it will be lovely to see their partner interacting with the baby, but they don't for one second consider how devastating it is to see them not. It just doesn't cross their mind as a possibility. People think that they are independent and don't need to lean on anyone, and they might well be, but once they are completely vulnerable and dependent and then find out that they can't lean on their partner like they thought? That reality check is like a pallet of bricks to the head. People think that a baby will make their partner grow up and take responsibility, and then they understand what that responsibility is, and their partner just simply can't handle it and just doesn't. And now it's all on them.
It is so much more complicated than "any sane woman wouldn't have a baby with an abuser".