Many people, for religious or cultural reasons, would see "getting rid of" an unborn child a considerably worse thing to do, then to have that child in less than ideal circumstances.
Some of it could be naivety, not realising how much a baby will affect your whole life. But nevertheless, it could be well be a "I've made the mistake, the baby shouldn't have to pay the consequences, so I will have to".
For strong biological reasons, the emotional attachment to an unborn baby gets stronger and stronger through the pregnancy, so the longer somebody knows they are pregnant before making any decisions, (or the later they find out) the more likely it is that they would keep the baby. Generally fathers won't have that emotional attachment as strongly, so it's much easier for the reluctant dad to tell his girlfriend to have an abortion.
Giving up for adoption might seem a more sensible thing to do, but the extreme emotional response that most feel to a baby they have given birth to, makes that an incredibly difficult thing to do.
and also, in spite of all the potential problems, many will feel the one who will look after that baby best is its parents - or parent if only one. Which is harsh on adoptive parents, but fair when you consider the chaos of the care system.
Also, to be honest, I think people judge girls more if they carry a baby to full term and give it up, then if they have an abortion. which to me is a bit weird, but there you go.