Why should they pay a higher percentage?
Actually, I think those figures are not correct. The figures I've seen say the top 1% pays 40% of all income taxes. The top 10% pays 70% of all income taxes.
I'll answer your question in a moment, but first I want to point out that you're using a statistical inversion fallacy. You're confusing two different measures:
1) What percentage of their income the rich pay in taxes; and
2) What percentage of total tax revenues the rich pay.
That the rich pay only 21% of their income in taxes, and that they pay 40% of all taxes, are not contradictory. Both could be true at the same time, if the rich hold a large share of the income, which is obviously true.
Now, why should they pay a higher percentage? Because, as you are prone to point out, a dollar is "just a worthless piece of paper." Its value lies in what it can buy. And so the value of a tax debt,
from the perspective of the individual taxpayer, is determined by what the money would otherwise be used for.
There is a hierarchy of things that everyone does with their income.
First, they take care of necessities: food, shelter, clothing, and those expenditures necessary to survive and make a living.
Second, they spend money on luxuries: things they could do without, but would like to have.
Third, they save money for security or to make a big planned purchase.
Fourth, they invest in activities that produce real wealth: businesses that grow crops, make goods, or perform services.
Fifth, they invest in financial instruments that amount to gambling: activities that don't produce any real wealth but may succeed in generating a profit by shuffling money from one person to another.
The poorest people never get past the first step. Working class people are generally confined to the first three categories. Middle-class people are also usually confined to the first three categories and sometimes into the fourth. The richest people put their money into all five categories, and the richer they are, the more of their money goes into the fifth category (except when, by temperament, they prefer to engage in charitable activities instead -- but that of course is tax deductible).
When money is taken by the government in taxes, the taxpayer cuts spending in these categories in reverse order. So a tax bill that, for a working-class person, would mean less spent on luxuries or less money saved, for a very rich person cuts only into his gambling money.
A rich person's money, in other words, isn't worth as much to him as a poorer person's. He can pay a much higher percentage of his income in taxes before he gets to the point where he's actually losing anything of value.
Taxes should be levied so as to do two things: defray government expenses, and do the least possible harm. And that means they should be taken mostly from the rich.