I attended Yale and it feels like everyone is pre-med or trying to get into finance (consulting or investment banking). You sort of feel out of the loop if you're not doing these things (I was one of the dozen computer science majors in my year).
A lot of these people don't see the problems until it's too late. The perception by society is that being a doctor is a stable well-paying job -- most people don't take into consideration, like you said, all the tuition costs, and years spent in school.
If you go to Yale, you're expected to achieve great things -- that's just the societal pressure. Not everyone deals with it well. So some hide in the "safety" of what society expects, rather than pursuing their dreams. These people would be too embarrassed to pursue their dreams, if it meant say, being unemployed for years or working a minimum wage job. I suppose that's more of a pride issue, but sometimes it's borne of a want to please others (while unfortunately forgetting about self)
That said, she's not even at the school yet -- she'll probably change her mind when she gets there. Plenty of people do once they're given a chance to think for themselves in a new environment. But plenty of people also ending up burning out because they've pursued pre-med or finance without it really being their calling, or love.