Slum is a comment about the quality of the structures. That happens when your civilization is poor and a typical worker’s wages don’t buy much building material. Marginal structures are a thing in rural settings too, only there it’s “rustic” instead of “slum.”
Anyway, we are not that fledgling economy anymore. Contemporary high-rise apartments are really nice! Even the mass-produced 5+1s are perfectly comfortable and dignified homes.