The same is true for political parties, religious organizations, and all sorts of other organizations that are made of people, including privately owned companies. Which of these concepts would you like to abandon, and what do you propose should be their replacement?
What you say is an argument for balancing power, not for concentrating it.
Government is also forced participation but I accept it as necessary part of a functioning society. I don't see Software Engineering Unions as a necessary part of a functioning society though.
Not sure how it works in the US, but from what I read you can't be forced to join a union there. But I think I get what you mean: once you work in a given place, there's usually not much choice which union you can join (or which will represent you in any case), and your only option is to change employer, location or career.
I understand that point of view, but I don't think it's a good argument. Employers also organize and wield their influence in ways that you seem to resent in unions. Don't like it? You have the same choices as above. (I don't think people should be forced into unions; I just think your argument is not anti-union, or at least cannot be fairly applied to single them out.)
The difference seems to be that the employer pays you and treats you well enough that you think you won't benefit from a union. Good for you! Hope it stays that way.
Unions are generally democratic, too, so there's also the option of voting for different union reps (or volunteering to be a rep) in order to change the union's policy or how the union works.
They quite literally are not, at least not in the US; half the states have right-to-work laws that prohibit this explicitly, and there are likely some protections even in the states that don't have a blanket protection.
It's been a concerted effort of corporate capitalists: buying & influencing media outlets, lobbying, offshoring, etc.
Unions were not able to make a compelling case. They had every chance to do so, starting with them being the default option in many industries.
America abandoned unions for a reason. It may have taken a concerted effort by corporate capitalists to get there, but that's not why. "Why" is because more and more people felt that the unions were not a net positive for them.