First of all, collective bargaining doesn't just have to be about monetary compensation. It could be about working conditions - e.g. regulations about being on "pager duty", ridiculous overtime during "death marches" (think game industry), employee training, etc. Remember that the vast majority of software developers actually do not work at extremely high-paying FAANG jobs.

Furthermore, on the topic of compensation, Hollywood has unions and yet top actors still manage to make 10s of millions of dollars per movie.