We tend to structure taxes with specific targets, rather than rely on general funds for everything. The idea being that public service funding is somewhat targeted towards those parts of society who take the most use of it. Gas taxes traditionally are how a good chunk of our road maintenance is paid for (a good bit comes from the general fund, too, but usually the gas taxes are restricted so they can
only be spent on road-related expenses).
EVs throw a wrench into the plan, and so the flat fee is one currently popular attempt to even out the taxes amongst road users. Another idea that got floated was tracking mileage on all cars every year and then levying taxes based on that. But this gets shot down pretty quickly because people perceive it as government tracking of their movements, and that is unpopular.
Personally I think we should just make commercial trucks pay all of it. They already have the infrastructure and policies in place to collect mileage-based taxes, trucks do the vast majority of damage to the roads they regularly travel on, and taxing them would spread around the tax burden to all the citizens who benefit from the existence of the road network (i.e. you get goods shipped on roads, you ought to contribute even if you do not own a car). Local roads should predominantly be funded through property taxes IMO.
Seems like in other parts of the world pigovian taxes are way more popular. They are extremely unpopular in the US. AFAIK gasoline is largely the same price wholesale across the world, but Europeans (as an example) are completely okay with paying more than twice as much at the pump and so more than half the retail price is a pigovian tax.