"so many" is a pretty squishy number.
You know, of course, that many don't choose it. That is, they are deserted or the biological father dies. Sorry, that isn't a choice, other than perhaps deciding to not abort if the event happens before that is no longer possible. Are you promoting abortion?
Hey, it was just a question.
But yes, some mothers do decide to not even inform the biological father, or inform for the purposes of getting child support with no intent whatsoever that the guy actually be a part of the kid's life. I would argue that as soon as the baby is out of her, that is not her decision to make - that he is entitled to know, period, end of sentence. If he is dangerous, then he should be in jail ... and he should STILL know he has a kid.
I would argue that failing to give due diligence in trying to inform the biological father should be a crime and should certainly be grounds for law suit (light on damages, heavy on emotional distress).
I can't see many feminists agreeing with me, but then they have never been concerned about what their actions mean to men. It's pretty much what defines the current movement.
And of course, some women choose to leave marriages, sometimes for good reasons, sometimes for bad, and yes, I'm sure that some would not if they didn't think they would get support from the guy or from the State, but that doesn't mean that support shouldn't exist. After all, do we really want couples to be forced to stay together when one or the other is abusive?
I expect "so many" would become somewhat fewer (though not dramatically fewer) if biological fathers had the right to give up their parental rights and responsibilities before the child could be aborted. That is, if Mom knew she was going to have to do it on her own, she might choose to abort - but most of those who would be willing to have the baby to begin with are probably going to try to have it even if there is no child support.
In any case, "why" is easy to answer: They want a baby. Period. Reproduction is one of the two most basic instincts required by evolution (the other being survival so you can get to the point of reproduction). They aren't looking to bilk men or society. They want a baby.
And it's amazing what they'll put up with to get one.
The most offensive scenario you'd probably come up with, a woman chooses a man with the right genes or the right resources, with no intent to end up with him, has sex with the intent of becoming pregnant, using deceit as necessary to make it happen, and then using whatever means necessary to make him pay child support while giving him as little access to the child ... yes, it happens, but it is *INCREDIBLY* rare. It's simply too risky.
What if he simply runs away? That leaves her with the child and no support.
What if he gets violent and kills her or the baby? It happens.
What if her actions become well known to the community around her? Ever heard of "The Scarlet Letter"? I believe women are far more likely to be scared of that kind of reputation than men would be.
Many are taking your question as to show that women are intentionally bilking men and the government to have a baby, and, though I'm not saying it doesn't happen, it is pretty rare. It is far more common, for instance, for men to be late or not pay child support payments.