Self blame preserves an emotional connection with one's family. It maintains a shared reality with parents, in which we all agree I'm the bad one.
Self blame avoids alienation and disloyalty (and the negative emotions that go with this) of seeing parents as abusive, critical, or deficient.
Self blame preserves the illusion of control. If it is my fault, then maybe I can stop it. I can become good enough that bad things won't happen.
Self blame preserves safety by becoming disentitled to self-expression that would increase the risk of further abuse. It avoids further attack. If you don't stand up, you can't be knocked down.
Self blame avoids a sense of loss and great grief over a limited or wasted life if you realised you were actually perfectly adequate and lovable all along, in spite of your parents' distorted beliefs and dysfunctional behaviours.
Self blame gives you permission to be incapable. You weren't able to be carefree as a child but now it's your turn. You can avoid the adult responsibilities of coping with demands and pressures by not being seen as capable, so you live your life as a defiant child.