Personally, I think its because without them the story would feel too bland, boring. Don't get me wrong, I like wholesome unproblematic manwhas as well but this types brings spice to the story. Not that being abusive is right or anything, it just gives space for major character developments that would make the whole story better and enjoyable.
And regarding about readers not being able to forgive the mc even if they have major character developments... Trust me, they will. Maybe not everyone, but most will. A great example would be Jiwon from BJ Alex. Dude was literally abusive to the uke. But after he got his character development and started developed feelings for the uke, a fandom was literally made for him. Although that one is not Chinese but you get the point.
Idk I just feel like there's a major difference between physical violence and someone just being a coldhearted selfish ass. I see the latter a lot in mangas and webtoons, but not the former. I only see that in chinese manhuas and it really just kills the romance for me. Like it doesn't matter how much character development the seme goes through, the uke will always deserve better than someone who used to beat them.
Like I swear every single BL and shoujo manhua I've picked up features a physically and emotionally abusive male lead. I get that there are most likely misunderstandings involved, but abuse is never justified. It just makes the male lead look like a terrible person for resorting to violence when under pressure, and I feel like readers will never be able to forgive him for that even when he eventually changes and falls in love with the MC. Bc you shouldn't HAVE to be in love with someone just to not abuse and torture them.
I just don't understand why some authors think violence is a good base for a romance story.