Unionizing a workplace is insanely difficult. It requires tons of bureaucratic work (to meet the standards set by state labor agencies) while also trying to get a large number of people who probably don’t know each other well, if at all, to all agree on a few things, all while risking their employment. This work often takes years. And all of this organizing work is unpaid labor on top of one’s regular job (which is already likely underpaid/overworked, hence the organizing effort). There is no “all they have to do is sign union cards.” The scenario you pose is practically impossible.