"Miles per gallon"/"kilometers per litre" and their inverses are more convenient when people will be integrating that information with "dollars per gallon" / "Euros per litre" to get the information they really care about: how much they will be spending on fuel. "Gallons" / "Litres", though they are volumes, are being used abstractly to talk about an amount of gas, in the same way we could have abstractly used mass to refer to the same thing.