Re: integration-- every single American city is de facto segregated. Even some of the most census-diverse cities in the country are set up in ways so that people of different races almost never mix. The US developed an entirely alternate kind of transportation based on the personal car once trains and other forms of public transportation were integrated by law. The only places where you really see much racial diversity are in places that have been integrated by law. Even then, those institutions that are de jure integrated do everything that they can to work the system (e.g. hiring/admitting seven black women for every black man).
When a game does the United Colors of Benetton thing, they are trying to reflect the propaganda of the system rather than what that system actually produces.
Not my experience. Maybe I just live in a much more diverse area (I live in Texas, where no one etnicity is a majority), but I currently, and always have, had people of different ethnicities in all the neighborhoods I have lived in, schools I have gone to, and workplaces I have worked at. One major reason I always roll my eyes at all the "race war now!" type folks on the codex. Somehow the places I have lived my entire life have managed to withstand diversity without breaking out into violent racial conflict, and even managed to be some of the best places to live and work in the country with massive growth.
And it certainly isn't because it is forced.
While there are plenty of places where there isn't much diversity due to one ethnic group being a very strong majority, there are many places where that isn't true as well as I have visited other parts of the country that clearly had a fairly diverse group of people walking around and interacting. It is also a very big country, so I wouldn't assume whatever personal experience you have had (or read about) are the norm across the country.
I think the extreme diversity often seen in fantasy games just doesn't make a lot of sense though, unless there is a very specific reason for it. People generally can't travel as easily in fantasy settings as they can in modern days, so there just wouldn't be that much traveling around and settling down different places. An area on the boundary between two different cultures might have a fair amount of mixing/interaction between the two (although after a period of time there will be a merging of the two cultures), and same thing with would happen in an area that was recently invaded or conquered.
But half a dozen different cultures all co-existing in the same place? You better have a very good explanation for that.