Hard for me to even speculate on this, because I don't like kids, never will, and am pretty sure that my hormones really aren't going anywhere anytime soon. (I believe the Magical Age 30 Clock does not exist in me.)
Still, my guess here is a combination of culture and hormonal urges. If we didn't have the huge driving desire to procreate, we as a species would not exist, or not in the capacity that we already do.
Assuming the teen got pregnant because she wanted to, well, she has essentially chosen her career: Mom. I know women can hold down jobs and kids at the same time, but that usually happens when she gets a career first and has kids later; witness how college grads tend to have kids much later than high school grads, and the delay is proportional to the quality/level of their education, and how "hardcore career woman" types often wait until 30 or 35 to pop babies. Someone having kids in her teens is putting all bets on her primary life's work being a couple of kids. If that's what she wants to do with herself, good for her, although I have no idea why anyone would want this to be their eternal fate, so to speak.
My other guess is that she had a very romanticized view of what it means to be a mom. Certain cliques, groups, whatever, emphasize the lovey-dovey parts. It's like how rom-coms emphasize the infatuation stage of love, not the "old married couple issues" part of love. Your friend might have seen kids as a great opportunity to raise a human being from scratch, someone of your body, blah blah all that. What she isn't thinking much about is the countless amount of time she will spend changing diapers, wrangling friends and clubs and schoolwork and all that for the kid, feeding it, spending all her money on it...so on. And thus, as you said, she ends up poor, bored, and her babydaddy has run off to live his life.
It could also be that some of these women grew up in cultures where having kids really early was a thing, so they were essentially pressured into it by culture and history. They may not have a choice, from their perception.