Because one political party is bought and paid for by the Unions and the other is gutless and timid.
Most, if not all of the growth in industrial America has been in the open-shop states where you cannot be forced to join a union. I have worked in manufacturing my entire career in both open and closed shop states. The difference is night vs. day in worker attitude and productivity.
We need a "Compete America" program in the model of the Sputnik program. Changing labor laws should be a first item in that program. Most people don't know that for instance you need a simple majority to vote a union in and two-thirds to get rid of one.