Simple - these girls think that they can sink their claws into a man forever by having his kid. That, and since it has become less taboo to have *edited for profanity children and since teen mothers get praised like the Messiah for having children they can't afford or properly care for, they realize it's a good way to get attention, be lazy, and get free welfare benefits.
But the baby trap is less effective when marriage is not involved, which is kind of funny. Without that legal document saying you are married, it's easier for a man to flee and abandon his skanky girlfriend and illegitimate child scot-free...whereas a man is more likely to get roped into child support if he gets a divorce.
I think it would be wise to revive the practice of making pregnant teen girls get sent away to a relative's home, birth the child and then force them to give it up for adoption. Teen mothers are lousy excuses for parents and are still children themselves, both in mind and in body - I wouldn't trust teens these days with a Chia pet, let alone a baby.
And marriage definitely is not as permenant as children. In a marriage, you can get a divorce and never see your spouse again - with the exception of alimony (if you get a lawyer who knows their stuff, you could end up paying little to no alimony). With children, there's debating over custody, the child's guardian demanding child support (which can be in very hefty amounts), and basically those two parents will be bound for life by that child. So, unless the parents kill their child, they will always have some sort of relationship - a kid is a far more permenant bond than any wedding ring or marriage certificate. And yet, far less thought is put into having kids than arranging a marriage.