In your example, when you move out of your parents house so your dad can't beat you anymore, the beatings might stop but the mental effect they've had on you remains. Those effects will, given time, cause other problems. Not guaranteed ofc, everyone's situation is different, but it's often the case. You need to deal with the effects to prevent them causing other problems. This can be a challenge if you don't even realize they're there, or don't think of them as problems, or avoid dealing with them because you hate them.

I'll use my life as a practical example. I was severely bullied early on in school. I essentially withdrew socially entirely, to avoid as much of it as possible. Depression set in, but at the time it was almost a blessing, because feeling nothing was better than dealing with it all. Fast forward to college--things are much better, I had friends, but I'm still dealing with a lot of self hatred, because you don't spend 7 years of people hating you and come out the other side with a positive self image. Still depressed, still having suicidal thoughts. Just now, for the first time, I had social connections, and so a lot of anxiety about not screwing it up. Dated someone, who ended up falling in love with her idealized fantasy of me, not actually me, while I'm struggling to feel anything through the depression. We have an ugly split, and I come away blaming myself for not being that idealized fantasy, hating myself for hurting her and not being able to feel. My self image got worse. That made me extremely uncomfortable with relationships and romance in general, but I didn't realize until I was asked out by a friend. She thought I was interested in her, and that caused me a lot of anxiety and self hatred for not being able to, guilt over making her think I was. There's now so much self hatred, guilt, and negativity associated with dating and romance that I had to suppress any thoughts about it honestly. Fast forward to a year or so ago, I can't handle thinking about being in a relationship let alone go dating. I moved to a new city for a job right after graduation just before the pandemic hit, so I didn't have any friends in the area when we started lockdown and working from home full time. Even more socially isolated than when I withdrew completely all those years ago in school. When things started opening up again, not only did I not have any friends nearby to do anything with, but the old anxiety about being social in person from the start of college was back. You see how all the issues compound on one another? Talking with my therapist helped me understand all of this. She made me reconsider all of those events, helped me peel away the layers of guilt and self hatred that wrapped them to see the reality of those situations, rather than the distorted versions that I was beating myself with. On my own, I had learned coping mechanisms, ways of thinking and acting to let me function around my problems, but I didn't even realize that they were fixable problems instead of just the natural way I was until I started therapy. I'm also on antidepressants, but it's like... imagine the central heating in your house breaks. Your immediate problem is the cold, not the broken heater. You might have coping mechanisms, like bundling up and using more blankets and drinking hot drinks, you might start antidepressants, in this analogy plugging in a space heater or two, but the real long term solution is therapy to fix your central heating.

Your comment in general wasn't offensive, just that one side note. Depression and mental illness can happen to anyone, it's not something that happens to you because you're not strong enough to resist it. You can't avoid or stop being depressed through force of will.