At 16, there aren't many places to go with other people your age to just hang out and socialize. You're too young for a bar. Restaurants are expensive. Malls are lame. Maybe a diner, but that's only 4-5 of you at a time, and you'll probably need to order something. And this can be a burden for a diner owner for taking up tables for the cost of 4 cups of coffee an hour.
Your parents can go to bars and restaurants and each others houses and socialize. But as a teenager, going to someone's house generally requires the permission of other adults, and so on, which isn’t something you can plan around independently. And in our teenage years, we're generally trying to figure out what this life thing is without the parental supervision. It's difficult to explore independence with our parents within earshot.
I saw a lot of this first-hand, even beyond my immediate peers, as a DJ throughout my teenage years. When my peers were looking to hang out, they came to me for options. If I couldn't provide directly, I would generally know who could. It was usually someone's house / apartment whose parents worked 16-20 hours per day (like my mother). My own friends and I would try to rent places out to throw gatherings, and we never had trouble finding people to fill the room.
This was all about socializing and having a place to go. We needed these places to go without the pressures of our home lives and adults therein. And when we couldn't find a place, we resorted to waterfronts, parks, street corners, etc. Wherever we could find to just be ourselves with our peers.
In college, for the first time for many, we have our own places: our dorms. Add to that more responsibility (for most), and a lack of income (for most), and what better place to hang out with your peers than hopping from dorm-to-dorm? Why go to a park or hang around at a bus stop when you can go one floor down to another dorm? You’re no longer required to go out into the cold to socialize with your peers.
In adulthood, there are plenty of places to go, though most of them cost money. So in early adulthood, we enjoy our freedom to go anywhere and just "be". So it's bars, and restaurants, and wherever. We spend what little we make enjoying the newfound freedom to go anywhere. Sure, we _can_ go to the park, be we don't _have_ to go to the park. We can afford to go somewhere else and enjoy each-other's company, much unlike the previous decade.