What the developers have done is say "Ok, we need some circuits"... and then came up with Seattle for the Americans, Rome for us Europeans and Tokyo for the Japanese.
Basicly they have take a streetmap of the city and picked out some streets that they felt created a good track with some good scenery ie Seattle has that long run down past the staduim and Rome has the run round the Colosseum.
These circuits do exist in real life as city streets... but unlike Monaco where every year they close the streets off and race... these circuits have never had that oppertunity... except in GT3 of course.