The way they sell this is by appealing directly to the people losing what amounted to very high paying but relatively unskilled jobs. Coal mining, working on an oil platform, working in a refinery, etc.
That's why people believe this rhetoric even though it seems absurd on its face. They see the faces of people who have lost jobs that is largely obscured from other parts of America because it's of the geographic distribution of signed paychecks to the American middle and lower class.
To defeat that rhetoric over the next 2 years, liberals and socialists need to find a story for how they're going to solve that problem and preserve a livelihood for folks suffering.
And to be honest, that's a wee bit frustrating because as a whole millennials are already tackling these economic challenges without the safety net of a massive lobby, while being told they're lazy and valueless, and often with crippling student debt for a poorly conducted degree. But a vote is a vote, and not every citizen can or will vote against direct self-interest on moral grounds.