Alan Reynolds of the Wall Street Journal:

The OECD study-titled "Growing Unequal?"-also found that the ratio of taxes paid to income received by the top 10% was by far the highest in the U.S., at 1.35, compared to 1.1 for France, 1.07 for Germany, 1.01 for Japan and 1.0 for Sweden (i.e., the top decile's share of Swedish taxes is the same as their share of income).

A second fatal flaw is that the large share of income reported by the upper 1% is largely a consequence of lower tax rates. In a 2010 paper on top incomes co-authored with Anthony Atkinson of Nuffield College, Messrs. Piketty and Saez note that "higher top marginal tax rates can reduce top reported earnings." They say "all studies" agree that higher "top marginal tax rates do seem to negatively affect top income shares."

In other words, higher tax rates have the predictable result of causing earners to work and make less, thus actually bringing in less revenue to the government. 

But for the left, this is unsatisfactory because it doesn't give them that delicious sense of "sticking it to the rich" that helps assuage their rage against the productive class. So they will take less revenue as long as they can punish people for being successful.