Reasons:
many men who are domestic abusers kill women who leave them. It’s widely acknowledged to be a very dangerous time.
being alive is generally seen as better than being dead.
https://www.leeds.gov.uk/one-minute-guides/one-minute-guide-domestic-abuse-risk-point-of-separation
reason two: someone who is physically hurting someone close to them is demonstrating repeatedly they don’t give a shit about the law and are happy to hurt people. The police are widely known to be pretty ineffective at stopping domestic violence and stalking and are generally only reactive.
if you leave, you can get legal sanctions telling the abuser to stay away. (Non molestation orders).
evidence suggests that this actually increases the risk to the person who has left and certainly someone who has already demonstrated they don’t give a shit about the law just what they can get away with a non molestation order is laughably ineffective.
Thirdly, if you have a child it is well known that the courts will not protect them from physical emotional or sexual abuse and in fact are likely to insist on contact even where there is documented proven physical or sexual abuse that the perpetrator has spent time in jail for.
there’s a presumption in favour of parental contact and if there has been abuse this will usually start in a contact centre where the contact is supervised but after a relatively short period of time (maybe up to a year) will go unsupervised.
so if there is a child the perpetrator can easily take revenge on the child and the courts do very little about this.
In extreme cases the perpetrators will kill all the children and then themselves. It’s known as family annihilation.
see eg this research paper.
https://scholarsarchive.library.albany.edu/cgi/viewcontent.cgi?article=1022&context=honorscollege_cj
so it’s not complex at all, it’s just that the system at the moment is set up so that particularly if there is a child the abuser has 99% certainty of being able to exact revenge in one way or another.
the system does not protect victims of domestic abuse and in many cases deliberately offers up the children as further victims.