Imagine you’re a stock analyst studying the big companies in a given industry. As you sift through the financials, you discover that one company spends twice what its rivals spend on operations, but its performance badly trails the entire group’s. Is that the stock you’d slap a “buy” rating on? Probably not. And yet, Americans keep investing in the laggard enterprise known as U.S. health care. Annual premiums for employer-sponsored family health plans have jumped 55% since 2008, according to the Kaiser Family Foundation. Life expectancy, meanwhile, has barely budged. Activist investors may want to call for a change in management.