Another reason this happens in my opinion: If you graduate from a good college with a marketable degree, you get sucked right into a cushy job at a large company. If you never got that degree and don't have that job, you're not giving up as much as the guy who's used to the status and money associated with the big company job. The opportunity cost of starting a company is less.
I've found this can be a really big "problem" as well. And it's not just the temptation of getting sucked yourself into a cushy job. It's that much of your network, your closest friends, also have that same hot degree and face the same temptation. Holding a startup together is hard enough in the best of times; when everyone on the founding team is continually bombarded by really tempting offers, chances are someone will crack. (Unless you're in a place where Silicon Valley where working on a startup is definitely "cooler" than that cushy job.)
My startup died when my cofounder got into Harvard Business School and I got into Google (we're both Amherst grads). 99.99% of America would not call that a failure. But the bottom line is that my startup's still just as dead.
It’s akin to saying that If you don’t have a degree you are more likely to be an entrepreneur because your odds of getting a good job in a big company are very slim.
I would have agreed with you had Bill Gates, for instance, never been to Harvard in the first place. If he had applied for a job at IBM and had got spurned. If Microsoft was something he came up with as a last resort after failing to bag a job elsewhere.
