After an upbringing where they had not responsibilities or proper guidance, suddenly when all direction is elliviated from their life, in regards to school, they jump to the next best thing... the instinct that drives them to have a baby, to have motherhood responsibility, and a sence of duty.
These girls likely have never had much success in their life, or any sence of achievement, and the strongest feelings they have had is teenage love towards some, usually oder, guy, who has promised them the world... The choice is made for them... They can't think of anything else to do! They don't think they can get a job, they have no drive to learn a trade, they just know that because other people can have kids at that age, they must be able to as well!
Add the fact that society, from the perspective of these people, doesn't do much to dissuade them! Telly programmes like Teen Mom don't portray these young girls lives as either very good, or very bad... It doesn't necessarily gamourise teenage pregnancy, but it doesn't say 'hey you girls, you are going to fuck up your lives!' It creates an image of a safe environment, where yes, there are draw backs, but at the end of the day it all works out! Easily relatable to another young girl in a similar position!
On top of that the state, (at least the UK does) does a lot to help out young family's. Its easily phesable now to raise a kid independantly as a teen just using money given in benefits, even as a single parent! In the 70's and 80's a teenage single mother wouldn't be able to raise a kid on their own, and it would either have to be put up for adoption, or they would have to find financial support from somewhere else, be it family or friends or other!
Anyway, apologies, I have put lots of thought into this myself! I have wondered, looking at my previous school friends, exactly the same thing, and watched quite a few programmes to form an opinion! (Watch 16 and pregnant, seriously, but watch from a psychology and sociology perspective. Think about how the show would look to a 16 year old girl, who doesn't beleive she has a future! Its actually quite interesting!)