The slice is the top students, who used to exclusively go to large companies. Now it's not so many. My impression is the top CS and Engineering students can successfully conduct a national job search.
The top students still mostly go to large companies. Compared to the number of top students going to companies like Google (who hired something like 6,000 people last year), grad school, quantitative finance companies, and several other options, the number of top students going to YC or otherwise starting companies is not a large number.
Among my own acquaintances it seems to have more to do with people's strengths and lifestyle interests. People who are strong techies but entirely uninterested in business would rather go to Google, or even to somewhere like NASA or Boeing or IBM or Intel. People who are a bit more into entrepreneurship, and/or don't like having a boss, would rather go into starting a business. Depends on the specific technology as well: a lot more web-tech people go the startup route than hardware engineers, and certainly compared to aerospace engineers.
