Q: Why is the FICA tax a percentage, while federal, state, and local gasoline taxes are fixed, named numbers?

A: Corporate capitalism is addicted to cars-first transportation, and must therefore increase its public subsidy over time.

This is especially true in systemically normal times, during which the masses lose economic ground to the overclass.  In other words, keeping the Bubbas driving is more important than keeping them fed, clothed, sheltered, insured, or anything else.