Why? There is no doubt in my mind that if you're the type of student who can get into MIT/Stanford/CMU/Berkeley/etc (at least Berkeley Engineering), you will earn more by not going to college (at least in tech hubs like SV and NY). Assuming you have a marketable skill (iPhone, Android, front-end, Django, Rails, etc), entry level (including internships) starts at $80k. If you don't have such a skill, you can teach yourself over a summer. In four years, you could be making $110k+ (very conservative). A CS grad straight out of a top college will probably make $93k (EDIT: revised to $95k, which is average for a Stanford/CMU/Cal CS grad) [1], depending on the market climate.
You factor in the anywhere between $0-250k tuition you're spending on college (financial aid depending), and the ~$380k you'd earn in 4 years, and, at least on the financial side, you come out way ahead if you don't go to college, anywhere between $380k to $630k ahead, just from those 4 years alone†.
In fact, this probably applies to most colleges, not just top-tier ones, but in which case the salary numbers are more variable.
You also won't be behind in terms of knowledge as long as you're proactive - the resources for self-teaching theoretical CS have never been more numerous than they are today.
Now, that being said, the reasons I'm attending college instead of dropping out (I did take a year off to work at a YC startup): I value the unique social experience of college, my parents are paying for everything (huge mitigating factor - if I had to pay for everything I would not be going), and I see the value in pushing myself out of my social, academic, and extracurricular comfort zones - hanging out with business students, doing debate, taking classes in literature, etc.
[1] https://studentaffairs.stanford.edu/cdc/jobs/salary-grads, http://www.cmu.edu/career/salaries-and-destinations/2012-sur..., https://career.berkeley.edu/Major/CompSci.stm
†Tax not included (subtract ~22%). The effect of having a higher salary due to work experience not included. Number varies depending on your housing costs. Number varies depending on student loan interest. Number varies depending on yearly tuition increases. The $250k figure for college tuition usually includes housing, your employment take-home money will depend on where you want to live in Silicon Valley (SF = big hit; East Bay = fairly affordable).