There are a million different ways to ascertain "income" to evaluate whether or not a person is paying their "fair share". That we doggedly refuse to do
any of them if your income is not subject to payroll taxes is a big reason why people (rightly, I think) don't trust elites.
To be honest, I have no special qualms with businesses doing their level best to minimize their tax obligations, provided they invest in their workers and/or R&D. Its just that they don't. The whole point of the old-timey ridiculously high highest marginal rate was to induce companies to do literally anything else than just pay their top executives more. Higher wages for the rank and file, investment in facilities, R&D, charitable giving, etc etc etc all have far greater impacts on society than the equivalent quantity shipped to the fed as taxes. If the ultra-rich want to avoid paying taxes by instead donating large sums of money to charities with broad socioeconomic impact, I have no problem with that either, for the same reason.
The issue is not that the ultra-rich don't pay their "fair share" (whatever that means), its that the tide that raises their ships does not raise all ships equivalently, and the ways they avoid paying that "fair share" disproportionately benefits only them. I don't really benefit if Jeff Bezos donates a lot of money to Jeff Bezos' nonprofit to manage his art collection or whatever. I do benefit if he just paid an equivalent amount into the general tax fund (or donated it to the March of Dimes or something).
I regularly pay out 20% of my annual income in the form of taxes. Its hard to be super sympathetic to e.g. Elon "I paid $11 billion in taxes last year!" Musk, when most years he pays far far less than that, and his net worth (as figured by his borrowing potential) grows proportionally a lot faster than mine does.