Because the average college student's brain doesn't mature until sometime around their middle twenties. That is why you will notice that on surveys, they group a part of the later teens with the early 20's. Then the later 20's are grouped with some of the 30's.
In my opinion, has to do with some kind of maturity concept that much of society doesn't grasp until a certain point of their age. So what does that mean with respect to major choices? They're probably in the major of something they want to do or their parents want them to do. They don't see the future, students only see right now. Leaving the whole socio/psychological reasoning behind this and just speaking layman's terms.
You can see a lot of IT majors who hate the job these days. Sumguy made a "what do you do for a living" thread the other day, you could have a look in there for some examples.
Personally I always thought I was going to grow up in IT. Second grade I started messing with dos, qbasic, win 3.11. College I took computer programming as my major, then met a mesmerizing sociology teacher which was enough to get me to switch majors. Then I graduated with a sociology impact and I started working IT, it wasn't easy getting research position jobs, but IT I still knew like the back of my hand.
Two years ago I quit the IT world.
It's all about making sure of your future, for your sake dude. Don't let those froot loops get the better of you.