[ezst]

Because teenagers are not known for having the best judgement? Because parenting comes with commitments and obligations that, early in life, conflict with other things that are seen as desirable for a well functioning society (education, equality, opportunities)? Having more experience in life generally makes you a better/better-prepared parent?

Don't read me wrong, I'm not stating that the current economic/social pressure is good at producing healthy/happy parents/families, it just doesn't seem good either to turn teenagers into parents. The "sweet spot" probably lies in between, and what you say is very much an appeal to extremes.