I don't think it is about just not building in valleys. There are two approaches to floods: one is to leave enough space for the river to meander, flood and so

I don't think it is about just not building in valleys. There are two approaches to floods: one is to leave enough space for the river to meander, flood and soak into the ground. The other is to regulate the river - dig it deeper, more straight, build embankments, basically making it an effective drain pipe. The second approach was popular in Europe because economy: it frees up valuable land (hence "building in valleys") and makes the river useful for things like transport. But it has downsides: it destroys habitat and efficient drainage here means a flood elsewhere. And then when you have superefficient drainage everywhere, it becomes a problem for ground water levels. So "don't build in valleys" is rather: don't over-regulate rivers to make buildable land.