You're thinking too intelligently, not politics and greed. Politics because raising the gas tax is a big no-no. Greed because that raise would be again undercut by the next generation of fuel efficient cars. Remember, they don't really care about fuel efficiency, they just want their money, but they can't say that or the eco-weenies would kill them.
but this new tax is going to be even more harshly viewed by the public, as opposed to a modest increase in the existing gas tax. and if the next generation of cars gets 20% better mileage, raise the gas tax another 20% at that time.