And if you've gone for a less wild world or a more wandering tale, world-building constraints can bind more tightly. Plus of course the need to provide appropriate settings for the incidents of the plot. Plus the need for variety, required for aesthetics, to provide contrast, and keep the readers awake. (Yeah, there are other things that can keep the readers awake. It's unwise to neglect any of them.)
Enormous cities out in the middle of nowhere need a reason. The pass through which all the merchants must travel. The only source of water in the desert. A port city would be less likely. The prevailing winds would have to be odd for there to be no farmland, and hence farms, and anyway, a port would have to be in an odd location to work as a waystation, which would be the only way to isolate it. Very concentrated and very isolated locations, lovely though the contrast is, do not make good juxtapositions on maps.
Culture shifts rarely, too. Long distance travel, whether airplane or dirigible, or starship or magical portal, is needed to get it juxtaposed. Indeed, places that are easily accessible to each other -- and have been so for a while -- are very prone to have very similar cultures.
Poor and rich can be contrasted more easily, especially in times when cities had to be concentrated enough for everything to be within walking distance, or at least riding a horse distance for practicality. And, of course, only those with horses -- or who could rent them -- found riding a horse practical, and many of those would send servants, on foot, to buy what was needed. Most people would have to buy stuff close by them, and that meant lots of trades, close by, and all sorts of people from the richest to the poorest.
Rural and urban, too, though many cities would not look very urban to us. For milk before there were milk trains, there were cows in the city. People kept pigs, and gardens, and hens, and other such practicilities where feasible. Still, the effect was confined. And since a lot of foods can't be moved far, there were farms close to cities.