Because in the parent/child dynamic, even when the child is well into adulthood, there tends to be a power imbalance in favour of the parents which also gives parents more responsibility when things go wrong. Also, the most public parents who've dealt with this like Doormat Mum No More author have largely shown a very aggressive side, particularly against the idea that her actions played a part in her child's choice. These are not true in all situations, particularly once the parents are elderly, but I think these are big cultural factors in why the public largely blames the parents on this.
I went no contact with my parents, both for different lengths of time, due to their addictions and not feeling safe being around them while my kids were young - both had made threats while intoxicated. I also left the country my parents and the rest of my family live in and have not been back in decades, so even before that and during reconciliation attempts, all contact has been email and phone.
After attempts at reconciliation, my mother chose to go no contact with me after a disagreement between myself and my sister - she deleted the email address which was my only way to contact her about 14 years ago, I've heard nothing from her since. I maintain the same email address we used to talk through in case she changes her mind. This has continued throughout multiple family illnesses and deaths. I accept that is her choice, I think it was just a final straw of things where our differences were butting against each other and she didn't want to try anymore. I wouldn't push her on it even if I could, I'd rather she'd continue with me if she wanted to, not because she felt she should - we spent a lot of time prior to that having a lot of people tell us how our relationships should be, having other relatives discuss how they'd hope we'd become closer and it never helped. Even as a child, we had other relatives saying this while there were months we didn't speak or see each other (father had full custody, she didn't want any). I accept I was never the child she wanted and I only hope she's found the peace she didn't have when I knew her. I don't see much around when it's the parent that pulls the plug on the relationship, I think it's probably more shameful for a parent to say that don't want a relationship with their child than a child to say that, parent are meant to be unconditional and all that, but lives are more complicated than that.
During no contact with my father, he would contact me at most once a year, by email, usually to wish me a happy birthday, sometimes that also included family updates or his thoughts on things. If he had contacted repeatedly throughout the early days, I likely would never had responded back or attempted reconciliation. Part of it was my needing to build my life safely without him or fear of him and that fear lasted a while - even as a young adult, he still seemed far more powerful than he really was. His very limited contact over the years is part of why we were able to reconcile - he showed some understanding of where things went wrong and that he wanted to amend and rebuild our relationship, and I in time dealt with my fears and could see another path. I admit at times during that I didn't want there to be another path - I was safe on the path with no contact and reconciling for most of it felt too big of a risk emotionally and otherwise. It was very raw in the first 40 days, it was still raw in the first 4 years, I'd say. It's been about 6 years now since we started reconciliation and still a work in progress.
Is blood really thicker than water? No, in either form of that line is idealistic at best, used to guilt people to maintain bad relationships at worst.
If I encountered someone who behaved like this our interaction would be minimal at best. Then why are you making numerous attempts to contact? Is it because you feel you should or something else?
I am aware that a better relationship would lift my associated depression I can understand that this is upsetting, it's likely more helpful to rebuild your mental wellbeing with the acceptance of voluntary association. Focusing more on the relationships you have that you want, rather than the one you now don't have with someone who you view as behaving poorly could be helpful.