There are two basic philosophies of tax rate structures:

1) Higher rates, with lots of "loopholes"
2) Lower rates, with few or no "loopholes"

Both generate roughly the same effective rate. Back when the nominal rate was 91% or 70%, the proliferation of loopholes reduced the effective rate for the "wealthy" and corporations to basically what it is today. Politicians in general (and democrats in particular) like option 1) because they can claim to be taxing the "wealthy" while holding on to the power (and campaign contributions) that come with being able to grant special interest loopholes. The Reagan tax rate decrease of 1981 and the Bradley-Gephardt-Reagan tax rate decrease of 1986 moved us very much in the direction of option 2). Europe saw that and did us one better, helped out by the revenues that they raise from consumption taxes. Both Bowles-Simpson and Domenici-Rivlin basically recommended completing the journey to option 2) (both advocated lower tax rates to increase tax revenues).

We could have a 15% flat consumption tax, a 15% flat corporate tax, and a 15% flat individual income tax, and have the lowest tax rates in the developed world in all three areas (save Ireland's 12.5% corporate tax) and generate about a $1T annual surplus, enough to pay off the federal debt in ~20 years. And that would be with a UBI and universal health care following the Bismarck model (which really wouldn't cost much more than our current woefully inefficient welfare systems).