Its possible only because human labor is cheap. And that is because those people have nothing else to do. Population is high, resources are scanty. People will do whatever they can to make a living.

But in most urban metros, cost of human labor is only going up. Washing machines are all over the places, dryers which were absent even a few years back are showing up. Robot vacuum cleaners are replacing few-hours/day maids all the time.

This is even more so in high rising apartments. Maid culture and personal servant culture in India is ending with urban living.