It isn't so much an assumption as a deduction - your repeated suggestion that folks only build where there's water seems rosy, and doesn't match with these facts, both current and historic. And, honestly, your phrasing seems a bit Eurocentric, so you may have been overlooking this reality.
Like, if you knew so many people lived without solid water resources, why would you argue that this would only be relevant where nobody actually lives? That makes no sense.
