I am sure some people leave after the first 'slap', but many can't/don't because the first 'slap' isn't the start of the abuse.
For me it started with love and trust which caused me to overlook them slowly taking charge of my life. They convinced me they knew better than me and manipulated me into doing what they wanted by rewarding me with kindness. When I stood up to them the anger came out, objects where thrown and broken. I was made to feel I was the cause of the anger, it was all my fault. My friends and family were sidelined as I tried more and more to appease the person I thought I loved and get back the kindness that became more and more elusive.
I left after the 'attempts' at suicide started, which where supposedly all my fault. But then the evil really surfaced. They had secretly engineered evidence that if I didn't come back they would make sure I never saw my children again, and I believed them. That broke me.
That was when it got physical, I had to be punished for trying to leave, it was all my fault. I wasn't even allowed to put my hands up to protect my face. And the sad thing was I felt it was still all my fault. I actually believed they were punishing me because they loved me.
Every day there was some made up reason to berate me, and the punishments continued until I admitted what I had done and apologized for it. It actually became easier to just make up some infraction and say I was sorry because the pain would be over quicker than if I tried to say I hadn't done anything.
I once managed to walk five miles in the middle of the night to a police station. It was closed. That was when the punishments became sexual as well as physical.
In the end I released I had to give up my children and I left for good. I lost my house, almost went bankrupt, spent 8 years fighting in the courts to keep my children, and had two mental breakdowns.
I dont tell people what happened to me now, they just dont believe it could have happened, why didn't I just leave after the first 'slap'.
I remember being on holiday with them on a remote cliff top, pretending I was so happy. I had my hands outstretched to push them off the cliff, no one would have known it wasn't an accident...
So I agree with the op, I wish people would stop saying/thinking I dont believe you because any normal person would have left at the first slap. Maybe then someone would actually believe me.