Look, if someone ends it after the first slap or insult, my hat is off to you.
But often the first slap is far from the beginning.
The abuse starts with eroding your self esteem and isolating you from support.
They teach you to think of them as doing you a favour by putting up with you.
They make you believe that standing up for yourself in the tiniest of ways is a huge misjudgement and causes them untold and unreasonable pain.
They persuade you that nobody else would ever look at you because you are such a worthless piece of shit while at the same time telling you that you can't be around anyone else in case they are interested in you.
They get you used to handing over your phone, keys, liberty because you aren't competent to be trusted with them.
They make sure that they have control of your money so you can't pay for a hotel or a taxi.
They isolate you from your family so you don't believe you have anyone to turn to for help if you do leave. They make you think that distancing yourself is a good idea, even your own idea.
They pressurise other people to join in jokes at your expense to reinforce the message that you are useless.
They hurt you 'accidentally' on purpose to train you that you expressing pain is hurtful to them and you must accept it without complaint. They also make sure you know that every 'accident' was your fault and suggesting that they were even partly to blame cuts them to the quick.
All the while they are doing this, they are also telling you that they love you and will look after you and there are flashes of the fun, attractive, kind person you were originally attracted to. You start to believe that, if only you could be a better person, there would be more of those lovely times you crave so much.
If all those things have been happening for months or years before the first slap, you are far less likely to be able to walk away. You will probably believe that it was your own fault or that it is a worthwhile price to pay to be in a relationship.
I get that some abusers aren't as good at it and don't get their victim to that point before the violence starts but many do and that is why even intelligent, articulate people, in high powered jobs, who may have previously judged others for being victims, find themselves unable to walk away.
Please don't ever say you wouldn't get into that position because, by doing so, you are apportioning blame to the victim and implying that they are somehow less clever, less strong, less able to look after themselves than you and that is not necessarily true.
People who have escaped those relationships are quite good enough at judging themselves for not leaving sooner. They don't need anyone else helping.