It's not. Relatively speaking, the rich do pay less % of their share of income in taxes, despite them obviously being way more able to pay taxes than the lower castes.
Warren Buffett infamously paid (and likely still pays) less of his income than his own secretary. This is beyond ridiculous.
And that doesn't even take into account the obscene gains in wealth that aren't taxed at all (partially, because taxing unrealized gains is hard). The rich just evade that tax (or practically, they shift the payment date towards infinity) by taking ultra low-interest loans on their stock and pay the interest from dividends.
[1] https://money.cnn.com/2013/03/04/news/economy/buffett-secret...