Because everyone did so, although some people said it wasn't good, most believed that the water in the village, coming from the wells, was groundwater, which was very clean and posed no problems when drank.
Even the cowherding kids, when outside without access to well water, would search for springs by the river— who knows if those even counted as springs, but in some spots by the river, as soon as a hole was dug that could spout water, it was called a spring, and they drank that water!
Not many fell ill; people were robust back then.
It wouldn't be right to say that there were no taboos, as there were still some. In the villages, there were various little stories, such as this one: A traveler, extremely thirsty when he arrived at a village, asked a local for water. An old woman in the village gave him a bowl of water and sprinkled some wheat husk in it, advising him to blow on it and drink slowly.
