Because they couldn't. The answer really is that simple.

Of course knowing that doesn't undo years of conditioning does it?

If my own parents can't love me why would anyone else?

That my parents are mentally unwell themselves just means that I should pity them. It wasn't their fault therefore I'm not allowed to be angry at them.

We both know that if it were another person in our shoes we would be nowhere near as hard on them.

It's hard when your feelings are beating you over the head and over whelming even ounce of logic you have.