But you only "observe" those who are "out there", not the ones quietly studying in their rooms or quietly doing more reserved activities. It's the same with anything, you notice the ones who are loud or disruptive as everyone else blends into the background. The vast majority aren't getting drunk and drugged up every day.
Take our local Uni. They have something like 13,000 students, maybe around 5,000 of whom are first years living on campus (most first years live on campus, second and third years move into private accommodation in town after their first year).
The student union put on a late night bus to "round up" the club goers in the early hours of the morning from the nearby city. One bus! So maximum of 50-60 people. Out of 5,000 living on campus. Yes, some will make their own way home, i.e. by taxi or by an earlier bus, some will stay overnight elsewhere. But I think it shows that the "problem" isn't that bad at all.
I'm sure that people will see large numbers of students on a Saturday night doing pub crawls and going to the night clubs, but it'll be a few hundred, not tens of thousands! Our city simply isn't big enough and doesn't have enough pubs/clubs for thousands of students. As far as I know there are only two proper nightclubs and both are pretty small, holding only 200-300 people! Remember, that's for a university of around 13,000 students!
In my son's first year Uni flat, 8 students in total, only 1 was regarded as a party going drug/drink taking student who was out a lot of evenings. 1 in 8! My son and the other 6 only went out occasionally, some more than others, a couple went clubbing every Saturday night, but other than that, they'd just go out for special events, like a birthday where they'd go for a meal rather than a pub crawl. He stayed on campus in year 2 with a different group of flatmates (randomly allocated), and the same happened - 1 or 2 went out regularly, but most of them would go out occasionally for birthday meals or for days out to the seaside or the zoo etc., maybe once a month.
I think it's classic Piretto's 80-20 - you "see" the 20% who are into the booze/drugs/party culture because they're loud and obvious. You don't see the 80% majority because they're doing "quieter" things like studying, going to uni clubs/societies, playing sports, or just playing computer games or reading in their rooms.