In some respects, America’s market for beer has never looked healthier. Where fewer than a hundred brewers operated a generation ago, we now can count more than 2,000, producing a mind-boggling variety of beers. Yet just below this drinkers’ paradise, we see a market that has never been more closed. Two giant firms — Anheuser-Busch Inbev and MillerCoors — now control some 90 percent of production. At the same time, a few giant retailers — led by Costco — are rolling up control over sales. This concentration is already diminishing real variety in much of the country.