Mmmm.... I hear what you're saying, but I don't think it's that simple.
When families blend, kids are torn in different directions. They mourn the loss of the nuclear family (even if that was dysfunctional and/or toxic), they are torn in their loyalties on top of which they have all the usual ikky phases that kids go through... like playing adults off against one another, and in blended families there are more adults to do that with.
The adults involved are dealing with their own issues. Guilt, anger, bitterness, perhaps feeling like a failure. Even the positive feelings like falling in love again, rebuilding lives, moving on etc whilst still trying to "deal with" the ex, and kids and their needs. It gets incredibly complicated.
Yes some parents do behave like children, but I doubt it's on purpose. Sometimes your own pain or bitterness or even fear clouds your judgement and you can't see how the way you're behaving is affecting other people. Sometimes, even if deep down you know that the way you're behaving is wrong, you can't seem to help it.
I'm not for a moment excusing bad parenting, and I do agree with you, the world would be a much better place if we could all put our own feelings aside and do what's best for the kids/blended/extended family as a whole... but parents are people, and people are flawed.
I agree, yes it's incredibly sad that people post about awful things that are happening, but also I see good things too. Forums like this are a place where people can get advice, support (mostly!), or simply a different perspective, and that's what drives change in the way we do things/think about things, and that can only benefit everyone.