It's called the ratchet effect. The government doesn't like giving your money back. That's why taxes almost never go down, either - e.g. the "temporary" income tax that only affects the richest 1% of people..
Both the marginal [0] and effective [1] tax rates have been trending downwards since WW2 with relatively little deviation from that. Pus this doesn't take into account all the ways wealthy people can avoid creating taxable income in the first place that generally aren't possible at lower income levels.
