If I wanted to reduce my water usage, I'd just take shorter showers and alternate days when I've not yet become smelly. These cost $0 and yield immediate savings.
Changing a person's behaviour, including your own, is pretty hard, its its less likely you'll actually follow through and take less showers.
Some people also live in households with multiple people, so that can affect your math as well.
In fact, disuse of water infrastructure can actually damage pipes and be very costly as opposed to just leaving the water running from time to time. (Just the logical consequence when the infrastructure – something that in the case of water can easily last half a century or more once built – is the main cost factor, not the water.)
Obviously this is not universal and there are also a great many places where water really is a scarce resource.