All you said is true...Not just the gas taxes but also the social security fund and any other cash-positive account has been raided by other agencies in the federal government. Numero Uno being the military.
However, we simply cannot just have privatized roads and bridges. That would be the death of interstate commerce (though it might put rail to better use if we can get away from the "just in time" manufacturing mentality).
You might like this outstanding presentation by the late Gore Vidal on the Military Industrial complex:
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