There are a couple of catches though. One is that we can't invest more at such rates of return. A VC fund may get a lower return per dollar, but they can invest a hundred times as many dollars at that rate. The other catch is that the $7 million doesn't include our operating expenses or the value of our time.
You could take a larger initial stake. I assume that the value placed on the YC program, including the cash, mentors, exposure, network etc has probably risen over time.