If you actually manage to get accepted to some Ivy, chances are you were at the very top of you class in HS. Excellent grades, excellent test, lots of extracurriculars, and all that.
The majority can't accomplish those things by just coasting through HS, and living a "normal" teenage life. Rather, it means dedicating lots and lots of your time to studying, extracurriculars, etc.
And it doesn't stop when you get accepted. So to me, it makes complete sense that some students get disillusioned - and start exploring other areas in life. It's not like the grind is going to stop when you start working - many of these ivy league grads go directly to the banking/consulting/law grinder that demands 60-100 hour work weeks.