I will never for the life of me understand why people insist on a progressive tax structure, not only is it unfair, but it disincentivizes seeking higher wages

I will never for the life of me understand why people insist on a progressive tax structure, not only is it unfair, but it disincentivizes seeking higher wages and incentivizes seeking tax-optimization structures. A flat tax with a tax-free base seems like the fairest, simplest, and most effective way of implementing a tax system (Estonia has been an extremely pleasant experience so far). I am aware this is also a "progressive" tax of sorts, but a much saner one. In fact, we all seem to agree that taxes work as a disincentive and we levy tax on the stuff we want people to do less of, but when it comes to wages and business we close our eyes and flail blindly saying "WE NEED MORE TAXES".