Because the river of gravy, by its nature, tends to flow out of its banks

Why a federal gasoline tax is a bad idea.  Well, it's another reason, along with the fact that Americans are just now seeing some relief from the sky-high prices of recent years.

Originally organized to finance the Interstate Highway System – a genuinely federal project if ever there was one – the fund now suffers from severe mission creep. About a quarter of its revenues aren’t even spent on highway projects, going, instead, to decidedly local concerns like mass transit or bicycle paths. According to congressional testimony from the Cato Institute’s Chris Edwards, that spending adds up to about $9 billion a year.
As John Marshall observed, “the power to tax is the power to destroy.” That’s why we regard any proposal for higher levies with caution. We believe that tax increases are only ever justified when the federal government can prove three things: 1) That it is only spending public money on legitimate public purposes; 2) that it is not spending public money on tasks better left to state and local governments; and 3) that it is spending public money efficiently. The Highway Trust Fund fails on all three counts.
Washington should tighten its belt and devolve financial responsibility for non-national projects back to state and local governments before taking more money out of taxpayers’ pockets.
We pay enough at the pump without having to subsidize Congress’ incompetence.

Get the mission creep straightened out, and then maybe we can talk.