Gig work is sweeping advanced economies, but workers on digital platforms like Airbnb and Uber make up just a small share of the independent labor force. About 27 percent of America’s working-age population engages in independent work, according to a McKinsey Global Institute survey, and the share is even larger in France and Spain. While only about 4 percent of the working-age population in the U.S. and Europe use online gig-work marketplaces, “rapid growth of the largest platforms suggests we…