Plus there is also the legacy fear that start-ups are a risk. Students aim for big companies and recognised brands as it's entrenched in their psyche that they must build a decent CV from the minute they step out into the big bad world.
I think the key point we need to make there is that as a developer you want to go wherever you'll learn the most, and start-ups credibly offer that - both by exposure to the broader aspects of building a successful business and also by density of talent.