why dont municipal water treatment plants used distillation to purify water?

Answer 1

I can think of a couple of good reasons.

1. Distillation would require too much energy to heat the water: expensive and wasteful.
2. Trace elements in fresh water would be lost to us.

Answer 2

I think it is used in some Persian Gulf oil states. Nobody else can afford distillation on a large scale. It takes heaps of energy.