If the sluggish U.S. economy wasn’t reason enough for tax reform, the ongoing IRS scandal demonstrates how a devilishly complex tax code enables government mischief. But the flat tax — a longtime policy goal for many on the right — isn’t the realistic answer to either problem. Flat-tax advocates see headlines about an out-of-control IRS as a chance to put the idea back on the public-policy radar in way it hasn’t been since the 1990s. “This is why you need a flat tax,” Wall Street Journal economics writer Stephen Moore told Fox News recently. “If you get rid of two-thirds...