Let me run through a few reasons why.

The Consumption Problem

When one interest group holds too much power, it succeeds in getting policies that help itself in the short term rather than help society as a whole over the long term. This is what has happened in America when it comes to tax policy, regulatory policy, and public investment. The consequence of channeling gains in income and wealth in one direction only is easy to see when it comes to ordinary household spending, which is one of the engines of the American economy.

It is no accident that the periods in which the broadest cross sections of Americans have reported higher net incomes—when inequality has been reduced, partly as a result of progressive taxation—have been the periods in which the U.S. economy has grown the fastest. It is likewise no accident that the current recession, like the Great Depression, was preceded by large increases in inequality. When too much money is concentrated at the top of society, spending by the average American is necessarily reduced—or at least it will be in the absence of some artificial prop. Moving money from the bottom to the top lowers consumption because higher-income individuals consume, as a fraction of their income, less than lower-income individuals do.

In our imaginations, it doesn’t always seem as if this is the case, because spending by the wealthy is so conspicuous. Just look at the color photographs in the back pages of the weekend Wall Street Journal of houses for sale. But the phenomenon makes sense when you do the math. Consider someone like Mitt Romney, whose income in 2010 was $21.7 million. Even if Romney chose to live a much more indulgent lifestyle, he would spend only a fraction of that sum in a typical year to support himself and his wife in their several homes. But take the same amount of money and divide it among 500 people—say, in the form of jobs paying $43,400 apiece—and you’ll find that almost all of the money gets spent.

The relationship is straightforward and ironclad: as more money becomes concentrated at the top, aggregate demand goes into a decline. Unless something else happens by way of intervention, total demand in the economy will be less than what the economy is capable of supplying—and that means that there will be growing unemployment, which will dampen demand even further. In the 1990s that “something else” was the tech bubble. In the first dec­ade of the 21st century, it was the housing bubble. Today, the only recourse, amid deep recession, is government spending—which is exactly what those at the top are now hoping to curb.

The “Rent Seeking” Problem

Here I need to resort to a bit of economic jargon. The word “rent” was originally used, and still is, to describe what someone received for the use of a piece of his land—it’s the return obtained by virtue of ownership, and not because of anything one actually does or produces. This stands in contrast to “wages,” for example, which connotes compensation for the labor that workers provide. The term “rent” was eventually extended to include monopoly profits—the income that one receives simply from the control of a monopoly. In time, the meaning was expanded still further to include the returns on other kinds of ownership claims. If the government gave a company the exclusive right to import a certain amount of a certain good, such as sugar, then the extra return was called a “quota rent.” The acquisition of rights to mine or drill produces a form of rent. So does preferential tax treatment for special interests. In a broad sense, “rent seeking” defines many of the ways by which our current political process helps the rich at the expense of everyone else, including transfers and subsidies from the government, laws that make the marketplace less competitive, laws that allow C.E.O.’s to take a disproportionate share of corporate revenue (though Dodd-Frank has made matters better by requiring a non-binding shareholder vote on compensation at least once every three years), and laws that permit corporations to make profits as they degrade the environment.