There's a pretty strong observation bias when it comes to college drinking. The social gatherings of basically anybody who does drink ends up involving drinking, which pushes out the non-drinkers, until you end up with two completely distinct social circles. To add to the effect, alcohol is wildly useful in socializing with strangers, so the non-drinkers know far fewer people.

Thus, if you ask someone who does drink they'll probably say "everybody drinks, except for a few people", and if you ask someone who doesn't, they'll assume most people don't, except for the minority of people getting loudly drunk in public. That was true even where I was at school, where polling found that 70% of the student body was drinking on the weekends.