WASHINGTON—U.S. employers added a modest 559,000 jobs in May, an improvement from April’s sluggish gain but still evidence that many companies are struggling to find enough workers as the economy recovers from the pandemic recession. Last month’s job growth was above April’s revised total of 278,000, the Labor Department said Friday, yet well short of employers’ need for labor. The unemployment rate fell to 5.8 percent from 6.1 percent. Many large chains, including Amazon, Walmart, Costco, and Chipotle, have raised starting pay to better attract applicants. So have other employers: Wages jumped in May for a second straight month, a sign of companies trying to attract more workers. And the average work week remained elevated, which suggests that businesses are working their current staffs for longer hours to try to meet rising customer demand. Yet even so, the number of people working or looking for work last month slipped slightly …