Because he has a distorted sense of entitlement to do so.
If you ask him not to treat you badly, if he agrees, he will see it as giving up a right, giving in and being weak.
The more he controls his 'woman' the better he is as a man.
It starts very, very subtly. with a
a look, a silence. a not so enthusiastic reaction. Then come the comments, light at first, little suggestions, then it gets stronger and stronger and stronger. teamed with the removal of you from your support network; all your friends are crap friends, they don't really like you, he'll cause problems with them, and your family too. He'll discount their opinions, make you see them as adversaries, not friends/family. Eventually his bonkers way of things, his skewed view, becomes the norm and you don't see it as mad anymore. Any time you question it, he'll gas light you back into line. "Of course that is not what I said, you ffing idiot.... " so you think OK, may be I did mishear...
All he needs in life is YOU, why do you not need only him?
If you don't please him he'll dump you, and who else would ever take you on with all your many faults
So you have kids, cos otherwise you'll miss your chance (see above), or he'll threaten to leave you if you don't.
That keeps you busy, and where he needs you, but the control continues, he carries on pushing and pushing you, testing you more and more and more, all the while making you terrified of being on your own.
The kids need a father, how can you do this to them?
That's kind of how it happens, it takes years though.