Because it's very easy and cheap to put a flow meter on your clean, full water pipe running into your house.

It's a royal pain in the rear maintenance nightmare to have thousands, tens of thousands, or in some cases hundreds of thousands of flow meters operating on much larger, partially filled pipes that can be carrying a wide variety of liquid and solid material running out of the houses for each sewer district. Far, far more expensive to purchase those meters and then to maintain them.