They are, just not directly, because the direct process for decertification or deauthorization is both highly risky (if caught, they could lose their jobs), and also pretty ineffective (the NLRB has a habit of overturning election results, rendering the whole process moot.

So, they use other tactics - hence the increasing support from union members for anti-union lawmakers. Political pundits often write about this as a "paradox" of union members voting "against their own interests", but it's actually pretty straightforward once you remember that union members are individuals who may or may not actually support the interests of the union that they belong to.