Taxes are such an important and partisan issue that they are instituted in ways that are deliberately difficult for future legislative bodies to interfere with.

Taxes are such an important and partisan issue that they are instituted in ways that are deliberately difficult for future legislative bodies to interfere with. Any sort of dynamic system would give some institutional body the ability to control tax rates without having to succeed in passing legislation, which is not something legislators want. Consider the Federal Reserve, which dynamically sets interest rates. Congress has very little input on this process (which is good!) and the tenure of the Chair is long enough that entire sessions of Congress can go by without Congress having any input at all. It’s hard to convince legislators that they should hand over power like this on issues that effect their chances of re-election, even when it’s correct.