Why do they refuse to take accountability?

I have only now realised how fucked the power dynamic of our relationship was only when I finished breaking up with her. My boundaries and mental health were pushed to the limit, all while she kept claiming how she knew she was a bad partner and wanted to improve herself for me, yet never did a single fucking thing to change her own habits and found a reason to pin the blame on me instead.

For context, a major reason as to my enlightenment of how fucked up she really was happened when we were having a long discussion for hours, yet a good friend of mine messages me suddenly speaking of how he was in a terrible headspace and needed someone to confide in and talk to. I do not speak to her for 50 seconds and I repeat, 50 seconds and she asks me why I am not talking. I apologise and say my friend is going through a really rough period and I would be diverting my attention to him for a short while. Her reaction is to break down crying and even proceeds to scream as loud as she can in a fit of anger or sadness, exclaiming how no one has ever made her their first priority and how no one ever loves her, completely disregarding the previous 3 hours we were talking for solely focused on her. In that moment I was just speechless, sure you might have your own problems but to break out in a childish fit of rage for not getting a minute of attention when someone is in a much more dire need of attention and support?

Pathetically unapologetic and afraid to take responsibility. If I had a dollar for each time they said they were "sorry" I would be a millionaire by now. They would only repeat that worthless phrase with no intention behind it and refuse to take action on any aspect that would fix or improve the way they acted towards me. When I tell her I don't like being emotionally manipulated she exclaims how she "feels trapped" because apparently threatening me with her claims of potential suicide is acceptable and a way she can "freely express her feelings". Never anything to fix herself and always a way to make herself the victim.

Funnily enough on the day I broke up with her she immediately entered a rebound relationship which only convinces me more of the disgusting lack of acknowledgement for her own faults and behaviours. I feel terrible and feel like the entire relationship was only so that I could serve as an emotional slave for her trauma dumping every single day.