Such is the case in most of the developing world. I.e only one city here in Mexico has potable tap water.

A RO system costs like $350, plus the filter renewal costs. I did the math, and it's cheaper to just buy the purified water off a purifier plant which is what most people do, unless you have a really large family. Also, a lot of places face severe hydric stress, making such systems unfeasible unless you have a large enough water reservoir to provide you with running water for a while. Most people don't have the space and even if they did, the low water pressure and or availability wouldn't allow them to get their reserves full in time, so they'll end up buying purified water anyways.