New York may be underrated as a startup hub, for the reason you cite. According to Wikipedia, "though it is not often thought of as a 'College Town', there are about 594,000 university students in New York City, the highest number of any city in the United States." Similarly, there's a lot of startup activity in New York, but it's not considered a startup hub because there's so much else going on in the city. Silicon Valley doesn't have anything else going on, so startups and the tech industry dominate its image.