The lack of a supportive environment for startups, and consequent lack of risk-taking, is an east-coast/west-coast thing. There aren't very many Amherst entrepreneurs. Most of my classmates did the minimum-wage "life experience" thing before finally going to law school when they wanted to settle down. It's like MIT encourages you to work really hard, Stanford encourages you to find your passion and then work really hard, and Amherst just encourages you to find your passion.
It's also worth noting that Harvard used to actively encourage undergraduates to take a year off to recharge their batteries and get a better sense of life outside of college.
The answer depends a good deal on the kind of MIT subculture you live in. I live in a place where people like to build awesome things, and frankly, I feel a lot more guilty about not making enough time to hack on side projects than I do about not taking more classes.
On the other hand, in certain other dorms there are strong communities of math/cs/physics olympians, some of whom regularly take 8+ classes a semester.
I got there and I asked my host and many of his floormates how they liked MIT and the response from everyone was pretty much "I love it, but there's so much work." And my host (a freshman, which meant he should have been taking fairly easy classes and that he was taking them all pass/no record) talking about spending 8-10 hours on the average PSET for one of his classes didn't help.
I really got the impression that it would be hard for me to find significant time to hack/start a startup, which isn't ok with me.
While MIT prides itself in admitting students with passion and interests over those with 1600 (I suppose 2400, now) SAT scores, I think that the truth is that the overwhelming majority of the students who admitted are still the type to overachieve, overcommit, and - most importantly - follow the rules. To get good grades in high school, you turn in all the assignments, complete tasks (as inane as they may seem), and take the tests.
What this turns into, from my experience, is a large set of students who are not natural risk takers. What's more, I'd argue that MIT makes it or made it difficult to take risks and explore on your own. They load you down with work, pressure you into taking on a UROP (undergrad research for the uninitiated), and it's a rare student who has a significant project on the side as well.
In contrast, places like Stanford seem to encourage thinking outside of the box / trying different things. Their culture definitely doesn't seem like it slants toward loading yourself up with classes as a badge of honor, and I think that's a hugely positive effect on the ability of students to find time to dream. It feels like a cop-out answer, but I've finally admitted to myself recently that this east coast - west coast cultural divide (see inmygarage, above) does exist, and is incredibly influential.
So while the students who have left MIT and become successful are well-known and, frankly, represent, their reputation is disproportionate to their number. You don't see* MIT sophomores walking around handing out business cards for their "startup" - and I don't think it'd be such a bad thing if that changed.
* in comparison to other schools