More importantly it is a significant source of calories. My brother quit soda years ago when he realized he was getting 1/3 of his daily calories from sugar - a little sugar is okay but you can't meet your daily nutrient needs when that much is from sugar. If it is just liquid soda is just as others (and probably better than alcohol), but there are other considerations.

> how much sugar water people buy, cases and cases

One confounding factor here is that oftentimes the price is only reasonable in bulk. I don't know about walmart, but around me the best deal typically is "buy 2 get 3 free". I rarely buy/drink soda, but on the occasions I buy at all I'll be getting many cases at a time.