Knowing vaguely how municipal plumbing works I find the idea that so many pipes and fittings could be clean enough to drink from to be utopian fan fiction

That's actually a super interesting topic! In areas with aging infrastructure in first world countries, they intentionally up the mineral content of the water so it forms a second wall so to speak on top of the pipes, keeping it much more sanitary. (Paraphrased). Primarily for lead. Generally though, the constant flow of water running keeps things much cleaner than you might think.