The 2nd amendment exists because every human has a right to sacrifice themselves meaningfully to fight tyranny. Guns are a proxy for power, and the 2nd amendment says that citizens have a right to some power. Since citizens have a right to some power, our government cannot get too tyrannical because there would be consequences.
The right to unionize is the right to use the threat of force (collective bargaining) against tyrants (CEOs), exactly like the 2nd amendment is the right to use threat of force (use of guns) against tyrants (people who rule as kings).
Unions are the representation of the idea that employees should be able to exercise meaningful power over their CEOs, in particular to demand better wages and conditions.
Without collective bargaining, why would a CEO ever have to compromise or negotiate except with other CEOs? If multiple companies acted like a cartel, how would employees be able to fight that without collective bargaining?
Why are wages generally proportional to the job, and not proportional to a companies profits?
Just like in software, the structure can be sound, while the implementation is poor. I think a lot of the problems people have with unions are implementation detail problems rather than structural flaws.