Roads are indeed often financed from the general government budget in addition to fuel, registration, tire tax. But many voters like having more roads. In effect the majority is subsidizing its own desires.

It's a self-reinforcing status quo of people who accept government spending on roads, traffic congestion, crash casualties, total cost of ownership of a car in the thousands of USD per year, physical inactivity, and air pollution. And they expect free parking and cheap gasoline.