Because companies are going to base salaries on collective agreements with the union which are usually lower compared to the salary high paying jobs have. At least that's how unions work in my country.
Unions are powerful because they can call for protests and employees who are in a union cant get laid off due to joining the protest. But its not worth if the job already has way better working conditions than like 80% of jobs out there.
I don't think your salary negotiation position is diminished by having some kind of a lower bound. Skilled employees should be able to get a higher salary if the employers value their skills, and that does happen all the time also in countries with strong unions, like those Nordics.
Even without unions Europe wouldn't have US tech salary levels, those numbers come from other market dynamics.
So play it out with me. Use Faangs as an example please.
Say all the engineers in all the top engeering companies by pay were suddenly in a union, how does the collective bargaining work on pay. You don't think pay would be lowered for high level engineers?
Collective bargaining would not apply to any of these salaries. Collective bargaining as we usually understand it sets the lower bound for salaries in a certain field, think of it as field specific minimum wage. Indeed, some Nordic countries have no minimum wage at all because ~all fields are covered by collective bargaining -- regardless whether the employees are part of the union or not!
So no, pay would not be lowered. I don't understand where this kind of a misconception comes from. Collective bargaining does not mean that everyone gets the same salary. As I said earlier, you are free to negotiate a higher salary, and companies wanting to attract top talent will still have to compete through compensation.
In the top engineering companies, collective bargaining could be used to negotiate other perks, like paid leaves, hour banks where you collect all work time exceeding a regular work day, paid parental leaves, and/or whatever topics are important for the work force. I'm not the one to define that though.
Collective salary bargaining is efficient in fields where the workforce is highly interchangeable, and the workers' ability to produce higher value is limited. If factory workers, miners, or nurses are not unionized they are really placing themselves in a precarious position.
"unions dont set max salaries" you are free to negotiate better salary. fact that union had agreed with company on minimum salary, does not change anything for you. yes the company can say "hey you asking too much, we talked with unions" but company can say the same either way "hey you asking too much, market is tough". so unions just the lower bar.
Unions are powerful because they can call for protests and employees who are in a union cant get laid off due to joining the protest. But its not worth if the job already has way better working conditions than like 80% of jobs out there.