As a Canadian who now lives in one of the US tech hubs, the reason I see at least is that Americans can't respect each other if their lives depend on it.
So density WILL make your life miserable. HoA rules, municipal laws, etc, are virtually not enforced. So you find a spot you consider tolerable, and you hope to god it stays that way. If someone decides to build a rental building near by, then it's basically over. You either learn to sleep through idiot screaming and people keeping a pair of saint bernards barking in a studio apartment near by, and frat parties that the cop will never do anything about or you move out.
Contrast that with up north, where I totally could call the cops on someone walking too loud at night and they'd do something about it (which virtually never needed to happen, since people were much more considerate. And I lived in quite a few places).
Now, I know what you'd say. In Canada/Europe/Whatever, it's just as bad! I had happened a ton of times!.
No no, these situations don't "just happen". It's a constant, multiple times a day thing. And it's a constant struggle between those who don't care and can't understand those who do.
I actually consider it one of the biggest reasons of the divide between the rich and the poor. You have the people with enough money to live on the 40th+ floor of a nice building in NYC, and those dealing with the stress, noise and neighbors at the bottom, thinking they're fine with it (but it probably drastically affect their sleep patterns, productivity, happiness, etc...they just don't realize it). The rich just "run away", because we can't agree on rules and respect them so we can live together.
Also, don't forget how terrible public transportation is. NYC has one of the best systems in NA, and it's not exactly fantastic compared to what you can see in many densely populated cities on other continents. Nevermind other North American cities (Hello Boston?)