We knew Lead was poisonous back in roman times. That didnt stop us from using its derivatives in paint, gasoline, solders (still used today), bullets and other common products. The disaster in Flint for example, was the result of the use of Lead and its alloys in the water pipes used there combined with poor decisions made in the name of cost cutting. Dont assume that people individually or as a society do dumb things only because they dont know any better. People do dumb things they know full well they shouldnt be doing all the time.