If there's some situation where taxes are high, but almost nobody falls into that situation, you might find that upon removing the associated taxes, people will structure their affairs to fit the newly created hole in the tax code.
Unfortunately, many complex systems are not created by any deliberate design, but by the accumulation of small decisions which often make sense at the time, but create something akin to horrifying tech debt over the course of decades.
Everyone agrees that the current system is a monstrosity, but there are winners and losers in any attempt to simplify things. The winners might be happy, but the losers will kick up a huge fuss.
But one of the issues with the tax system is that it’s not just a revenue generation mechanism. It is also used to encourage ‚good’ behaviours (e.g. saving for pensions) and discourage ‚bad’ (e.g. gambling).
So, whenever you make a change to the tax system, you are not just changing revenue generation, you are also adopting a political position. Are you „hurting frugal savers” by removing their benefits, or „supporting gamblers” by removing their taxes.