Say you are a talented employee at company X. You have invested time and effort in a project for the last three years and done good work. Now you get into a dispute with a manager over the future of the project. They start playing office politics and eventually get your project defunded, blocks your promotion and requires you to sign a non-compete to get your bonus.
While you can just quit and join company Y the first thing they will ask is what you did the last three years, what your project was and which position you attained. Chances are you are back in a similar position, having to reinvest that time and effort. Thing like this can really mess up your career.
If you are just working on a average project, not really expecting a promotion or in other ways are average, you don't have as much to lose. Sure, if you don't get treated right at company X, you go join company Y in a similar position. On the other hand since you don't expect a trajectory attaining things like good pension, vacation and overtime early matters a lot.
No, that isn't really how it works. Unions are basically insurance. Some of that insurance is minimum wage, but others are vacation, promotions, non-disclosures, arbitration, work environment etc.
Minimum wage in white collar unions usually isn't very high. It is usually just protection against the company dumping salaries below the level of reasonable living conditions.
> That means that not so many top employees wound want to join unions, making employers even less likely to employ union workers.
Talented employees have the most to lose by being unfairly treated. To be underpaid, to have your career interrupted, to not be able to take other opportunities etc. Again it is insurance.