Even if you take a Western lens to it and say ATLA takes place around 1850 that still puts Korra in the 1920s (and the air nation falling around 1750) and the only thing that really would be innacurate for comparing their timeline to ours is the lack of trains connecting the cities, at least in the earth kingdom where theres a lot of land to be crossed.
I suppose if you assume that Ba Sing Sa is self-sufficient with it’s large farms in the outermost ring then it makes sense from a tactical perspective to have fewer points of entry to the city. You’d also have to assume there’s either significant brain-drain from the villages into the city or that the villages keep to themselves so much that they have no need for better transportation between them
I’m not finding any good sources right now but some of the earliest trains were actually a singular railcar on wooden rails pushed by 1-2 people in much the same manner that the trains in Ba Sing Sa do, just sans Earth bending of course