> Unions are not similar to companies because they don't compete on the free market. For this reason, much like all state-funded institutions, unions are much more prone to corruption.
The government has intricate voting protections for organized capital: oversight of the voting process with minority shareholder rights, stringent rules for the board and corporate governance, allowed cross-company collusion through mergers with very little checks, especially if the merger crosses industry lines. And they get extreme protection from liabilities for damages they cause.
For organized labor there is little in right-to-work states: "minority" voter rights that say anyone can defect from the majority, in many right to work states the majority can't even freely negotiate a contract that says new hires will be bound to the voting process (each new hire can defect), most of the voting rules there just make things almost impossible to organize as a whole rather than protecting the equivalent minority stakeholders, and collusion between unions isn't possible in the same way due to federal laws making secondary strikes illegal.
Organized capital gets a great structure to collaborate together that would be illegal if they were owners of separate businesses, workers get forcefully atomized even if they try and set up the organization through a freely negotiated contract (due to freely negotiated contracts not being able to set terms for new hires, through the outlawing of "Union Security Agreements" https://en.wikipedia.org/wiki/Union_security_agreement). So things like dues don't have to be paid by new hires but the get the protections, then the collective action free-rider problem takes over and eventually dues for funding things like support during strikes dries up.
Imagine if new shareholders who bought some shares through an existing holder didn't have to be bound by the share-majority vote and could just sandbag mergers etc. by not agreeing to go through with it for their portion of the shares and they couldn't be forced to through the normal state collective action enforcement mechanisms that shareholders today all enjoy.