En masse is one of the millions of useful expressions stolen by English from other languages. (We call it “borrowed” or “loanwords,” but we’re not going to give them back!)

It doesn’t literally mean “in mass,” which would refer to measuring weight or volume, or could mean being in a Catholic church during a service.

If you wanted to use a different expression to denote the whole of the working population acting in unison you could, but “en masse” carries some appropriate French revolutionary connotations and avoids the communist implications of “the masses,” which could hamper recruiting.