PHOENIX—All around this desert city’s sprawling metro area, low-rise office parks with tinted windows and vast parking lots stretch to the horizon. This is America’s back office. Abundant land and cheap labor made Phoenix a premier place for companies to stash lower-paid office workers who don’t need to be physically close to clients or headquarters. The cubicle-based jobs—customer service, data entry, payroll processing—created a vital ladder to the middle class, helping replace factory work lost to overseas competition.  Now, these white-collar jobs are fading, too, thanks to continued offshoring and, increasingly, artificial intelligence. Tens of thousands of local workers suddenly…