I have been to a lot of countries, yes. I'd say the only countries where I've consistently seen more unsafe driving have been in SE Asia. Specifically, Indonesia and the Philippines. But I haven't been to NZ, so I can't speak to your driving culture.
From a quick look, NZ generally posts limits of 50km/h in cities. It's not uncommon to see 45mph (72km/h) limits in similar cities in the US. Factor the "10 over" rule in, and there are a lot of Americans who are used to going ~89km/h on urban roads.
P.S. to your edit: I agree! I think American road culture is more than just speeding, and that the things you've identified play a substantial role. That's why I said "neglect" rather than focusing on just speeding.
Our speed limits are actually similar for similar designed roads in urban areas, 50km, 70km, 80km and 100km. A lot of the road network is 100km national speed limit, and you wouldn't typically drive that fast on US roads of a similar design (single lane each way and quite often just a single lane for both directions!). But this is because NZ has a low population and pop density. While the US has some places with a low density, it can still afford to road up those areas "better" than NZ can.
From a quick look, NZ generally posts limits of 50km/h in cities. It's not uncommon to see 45mph (72km/h) limits in similar cities in the US. Factor the "10 over" rule in, and there are a lot of Americans who are used to going ~89km/h on urban roads.
P.S. to your edit: I agree! I think American road culture is more than just speeding, and that the things you've identified play a substantial role. That's why I said "neglect" rather than focusing on just speeding.