I'm not confused about how urban space works.Those places weren't blank, you as the player just didn't know about them. The universe didn't invent them for you as you approached.
But given that (i) the fiction of a RPG might occur in urban areas, and (ii) it is literally impossible for someone to worldbuild a fictional urban space that has the detail and variety of an actual urban space, then either (iii) the RPG setting will be impoverished, or (iv) some other approach is necessary.
An example of (iv) is found in Traveller (1977), as per my post just upthread. That approach depends upon "leaving blanks".