Because currently there's a employer PR, services like glassdoor, etc. but still not enough power to influence or avoid future bad faith executive decisions. I think the case of google's work on a defense project counts. When I was a fresh grad, Google was the great, good go-to place, it has changed. Given how big it is I don't know what effect Unions can have, and I hate the negative nepotistic and other effects of Unions, but an argument could be made, google would still be true to the "Don't be evil", if there were unions.* P.S: Admittedly it's just an argument that may not be tested/testable.