That’s the question Francis DeLuca asks in this Civitas piece.
One reason is that unions put up as many roadblocks as they can, such as having a narrow “window” of time in which members may withdraw. Such a scheme was recently struck down in Michigan for teachers, but unions will keep doing that as long as they can, until ordered in each case to stop.
Another reason is that when a worker decides to drop union membership, he or she also drops the right to have any voice in what the union, which remains the sole bargaining representative, does.
One more reason is that, while harassment is illegal, unions often find ways to make life unpleasant for those who say they prefer to stop paying dues.
The solution that cuts the Gordian Knot here is to repeal the authoritarian (and unconstitutional) National Labor Relations Act and return to the common law. Under common law, no one could be compelled to accept another person or party as his agent; the NLRA overturns that sensible rule so as to give unions more power. That’s a bad law.