Because at some point you have to move beyond the theoretical and pick a method - that was mine. My Corolla will get around three times the MPG as a Hummer, so basing the tax on that will get us in the ballpark, which means more accurately applying the true costs. It does not have to be perfect.
But the existing tax per gallon, charges the hummer driver for three times your rate. It also taxes the driver who is wasteful in the number of trips they make. It also saves money to those that car pool. Why is the existing system of tax per gallon not meeting your desire?