Why do some parents selfishly treat their children like buddies, gossip to the child about adult matters that children have no business hearing about, and try to involve the child in their problems?

Because there are some people that don't know or respect boundaries in terms of what is appropriate behaviour.  Not EVERYone has common sense or a sence of logic.

Especially when it comes to things like an estranged parent saying very derogatory remarks about the other parent to the child.  In most cases of two parents becoming estranged because they simply detest each other, and want to use the child as a battle ram .. It's typical for them to hate each other more than they love their child.

This is a common type of parenting, most likely self absorbed parents.