It's partly a climate thing. Flat roofs in areas with snow are begging for all sorts of leaks and general snow load problems. There are probably houses in the Northeast built with flat roofs. They probably also have leak issues. Heck, houses with sloped roofs often have problems at junctions and with ice dams. Commercial properties are, in part of the reasons you cite.

Lots of places in the northeast with flat roofs. Particularly dense city centers where snow falling off sloped roofs poses a danger to busy pedestrian sidewalks. As long as you keep your roof maintained it’ll last a long time. And “flat” roofs are always angled at like 1-2 degrees to ensure proper drainage.

I don’t know where you are but in Seattle almost every new house has some sort of boxy flat-roofed design

I've always assumed that the boxy things were more about maximizing saleable square footage and less about architecture. Also, "rooftop balcony" is a nice price adder!

I don't hate the boxy things, I just hate that there's nothing else being built in Seattle right now except boxy things. And they're not much to look at.

Here in Germany next to all new apartment buildings (usually 3-5 stories) have flat roofs.

Alas, in the US we are not as wise to the ways of steel-reinforced concrete construction as you guys are :(

I think (hope) that in Germany we are still using brick and mortar construction? Or at least some modular wall elements.

At least I have not really seen those rebar and concrete skeletons that are a common sight basically everywhere else.

3-5 story buildings in the US are usually just wood frame. Any brick or stone is just a 1-2” thick panel they hang on the outside of the building.

Yes, a common construction are these five-over-ones, with a steel-reinforced concrete first floor (for retail) and then wood framed 5-story apartments above.

This podcast goes over what an absolute disaster this type of construction is [0], but for apartments in most major US cities west of the Mississippi, it is hard to avoid _some_ sort of wood framing.

[0] https://www.youtube.com/watch?v=xVodkE47aLw