If you're referring to abuse as only physical then that may be the piece of the puzzle you're missing. Abuse doesn't tend to start with just physically hurting someone, there is often a long period of time of manipulation that gets worse over time.
Things start off innocent enough, you're in love, everything feels wonderful. Maybe they're a little jealous if you spend time with others, but it's just because "I love you soooo much, I miss you when you're not around" or "it's not that I don't trust you, I know you would never cheat on me, but I can't trust other people, it's just because I love you more than anything and can't imagine life without you."
It doesn't sound so bad. It's all out of love. They end up controlling you, your life, and your body through various methods of manipulation, often blamed on their love for you. They find fault with everyone close to you, whether it's a friend that just annoys them too much, or your family that just makes them feel so unwelcome and they don't know why when all they do is love you. And let's not forget the classic "they just don't understand what we have, they're just trying to tear us apart" and the next thing you know you don't talk to your friends and family anymore, but thats OK, you have each other.
These are just some examples of the way conversations could play out of course. There are many ways to manipulate someone, especially if you're good at it. Add in that abusers tend to target people who already have low self esteem and other issues and it's a recipe for disaster. Children who grow up in abusive homes are more likely to end up in abusive relationships because they already have a skewed sense of what relationships should be.
Sometimes you can be worn down slowly over several years before any physical abuse even starts, some people don't start until after you're married. And even then it'll turn into "why do you have to make me so angry that I lose control and hit you?". A rational person would think it's crazy to believe that you could be responsible for someone else hitting you, but someone who has been subjected to emotional abuse, manipulation, gas lighting, etc for years isn't going to be able to see how it's not their fault so easily.
And then of course once you're in so deep, leaving can become difficult and dangerous. Restraining orders aren't always easy to get and even then it still takes time for the police to arrive when one is broken. Not to mention when someone threatens to kill themselves if you leave them it makes it difficult, as well.
I know I've probably done a terrible job explaining, I'm very tired. I met my abusive ex husband when I was 15. We got married when I was 18, and I divorced him when I was 22. Looking back of course I can see all the red flags now with the benefit of age and wisdom, but living through it wasn't as clear cut. By the time we got married the only friends I was allowed to talk to were his friends, and only when he was around. Being so isolated makes it easy to control someone.
You're welcome to ask for more explanation, though I'll be more eloquent after I've had some sleep.