Is it avoided out of convenience? Is it assumed that there are magical ways to remove the refuse and the potential illnesses associated with it? How clean is the water in these urban areas? Are the churches and temples constantly casting purify water and cure disease to keep these environments running?
Beer may (or may not) have been the alternative to clean water back in those days, but it wasn't nearly as strong as the beer we know.
Do we just assume that in our magical worlds, refuse and water don't mix and refuse and open sewers do not lead to illness and disease?
Hand wave or magic or make it real? What's you take on it?