Trondheim also has a university (which increases the odds of a party happening), and one could also walk across the entire city in less than an hour :). Most cities in the US suffer from being designed around cars, but that has not changed in the last 50 years, so I don't think it explains the decline.
It's been years, but I hope Den Gode Nabo is still fun.
Part of the reason you can walk across the city in this time is because it is really walkable. I'm from the Midwest - Trondheim is the biggest city I've lived in but more walkable than any of them. I'll add that the "across the entire city in less than an hour" isn't as true as it once was, especially when you consider that places like Klæbu are part of the city now and the population has grown. Byåsen would take me over an hour to walk to.
Den Gode Nabo is still about, but its been years since I've been there :)
Other places had taxis (that you couldn't order ahead of time to get to work on time) and some had none until they uber/lyft. (Don't know the current situation).
I'm going to guess the other thing Berlin has is safe areas to walk. I can go to a party and walk home, safely on walking paths complete with shortcuts, without even being harassed by the police and risk getting arrested and in jail for the night (for public intoxication). None of these were luxuries I had in the states.
And I'll say that yes, I've been in some small apartments - but only some folks with small apartments can host. You probably have no clue how many would host if they only had enough space, but a small apartment with 2 adults that have hobbies limits things.