That's because you have no room for cheap housing to exist on, and cheap/low quality houses are usually built where the land is cheap. Simply put, land is at such a high demand or premium there, and its rare to see a situation where you'd spend so much for the land and not bother to put at least home of equivalent value. You don't have much land to waste on a trailer or projects. Here you can get a parcel of land for a couple of thousand, so someone will put a cheap home (trailer) on it and it makes sense, but you won't often see a trailer on land that costs as much or more than the trailer. So the economics of it just work out there that anyone who would live in a trailer in Europe couldn't even afford the land if they couldn't afford a house. There if you can't afford to build or buy a house, you certainly could not afford the land to put it on.
On the commute you're assuming that because driving 30 miles down the highway is easy you don't also have to cross through the city once you get there to get to the office. Take that 30 mile commute, say that takes 20-25 minutes to get 25 miles of it down the highway to the city, now you're in the city, so now you add the 30 minutes it takes to get through those last 5 miles now that you're in the city dealing with the same traffic as a city resident. Now you have the reason why the commute is worse. You still have the same commute through the city, unless the office and is on the edge of the city and you live on that end as opposed to the opposite side so you don't have to go through or around.
If the city is ringed by suburbs, being in the city center gives you access to potential employees from all the suburbs, not just the nearest ones. As well as employees who would prefer to take transit to work rather than drive (a growing demographic). Plus if you're trying to attract younger employees, they'd generally prefer to work somewhere that's convenient to the same places that are convenient to their friends' offices and close to bustling after-hours businesses. They want to meet friends after work, and would rather travel 15 min within the city rather than 40 min back to the city or in a ring around the city in order to do that.
The rise of two-earner households caused more concentration of employers in urban areas. If there is only one earner, you move close to that earner's job. If there are two earners, both need to be able to get to work reasonably. Thus, there is a network effect for employment.
Most aren't actually. Pharma (and now GE) aside, it was very uncommon in Boston for a variety of historical reasons. (Companies that were in Boston/Cambridge were actually moving out over time until quite recently.) Nor was the city of San Francisco (as opposed to the South Bay) a tech employment center until the past few years when it started to change.
But certain urban centers have become popular living locations for (mostly) people right out of school. They may also be near customers, especially in finance and related industries. (Even when engineering offices were in the suburbs/exurbs, it's always been fairly common for enterprise tech companies to have downtown sales and consulting offices.)
On the commute you're assuming that because driving 30 miles down the highway is easy you don't also have to cross through the city once you get there to get to the office. Take that 30 mile commute, say that takes 20-25 minutes to get 25 miles of it down the highway to the city, now you're in the city, so now you add the 30 minutes it takes to get through those last 5 miles now that you're in the city dealing with the same traffic as a city resident. Now you have the reason why the commute is worse. You still have the same commute through the city, unless the office and is on the edge of the city and you live on that end as opposed to the opposite side so you don't have to go through or around.