Water meters are installed in a variety of places - while some might be in the building, plenty will be in a pit at the street.

Even if it's in the same building, you now have the builder/homeowner's electrician needing to supply power to the meter that could be on the other side of the house from the electrical service. Should it be supplied from a dedicated circuit for reliability? If it's a retrofit, who pays for the wiring - and plaster/paint for the walls if it wasn't an easy run?

What if the owner turns the power off and goes away for a month while leaving the garden sprinklers on? Is that OK?

Electrically powered electrical meters work fine because if the meter isn't powered, nothing else is anyway. And it has to have all the safety and mechanical features for permanent connection to and safe usage of mains power, which adds substantially to the cost.

Trivia: the energy consumed by electric meters is not negligible, though it's fairly small. The meter is designed so that it does not measure its own power consumption - the power company pays for that, not the customer. Should the customer pay for the water meter's electricity consumption.

Extra trivia: A suitably efficient turbine could provide the watt or two of power a meter needs to transmit by leaking about a teaspoon or two of water down a drain.