You are correct, but only at unrealistic levels of bad. A potential competitor risks that they lower the prices after the have dug in the pipes and that is enough that the competitor is bankrupt. Even if water quality is the issue, it is easier and cheaper for the incumbent to fix the issue than for you to dig in new pipes. Only in the case where the incumbent has worn out pipes that all need to be replaced at once (unlikely - pipes last a long time and we have had good success relining pipes in many cases: leaving only a minority that need expensive replacement at any one time).

Because the bad has to be unrealistically bad before someone can compete nobody sane will.