Tap water is not just water. Water has a bunch of other things in it besides water. The main reason I can think to choose a different water supplier than the cheapest is the water quality. If the cheap supplier is gonna crap up all the water for everyone else, what's anyone incentive not to go with the cheap supplier? Doesn't that pretty quickly devolve into a monopoly?
With electricity, it is just electricity and people have opinions about how that electricity is generated. But even there, it's usually not a good thing. It leads to high pressure, scammy sales tactics that result in higher bills for no benefit.