Because mothering, like all traditional "women's work," is devalued, and fathering, like anything you need a penis to do, is devalued even more so. The act of actually raising the children you chose to bring into the world, rather than shunting them off to strangers, is considered woefully boring and stupid.  Along with that we have the idea that to say no to a child is borderline abuse and will damage their precious "self-esteem," and that expectations and high standards aren't even borderline but straight abuse, period. No one is allowed to have standards or expectations at all. Children are babied and spoiled and treated like little adults everyone must listen to--experts in politics and social good, despite never having lived under their own roofs or earned their own livings--hence the left's push to lower the voting age to 16. And many parents still operate under the desperate and insanely childish desire to be seen as "cool" by a bunch of people who can barely drive, and to be their kids' "friends" instead of their parents.  tl;dr our society has completely abandoned the idea that there is value and responsibility in being a parent, and that raising a child takes two people.