Unions are helping professionals serve people better and maintain standards. These psychologists are highly educated professionals. They are entrepreneurs. Part of the answer is easy.
Teachers are professionals, and they have been unionized for almost a century.
Members of the nursing profession have been joining unions for at least 70 years and have been in the AFT for two decades. Professionals are just as interested in decent working conditions and decent wages as other workers. They don't work under a contract or engage in collective bargaining. Psychologists are a perfect example. Like nurses and doctors, they find that managed health care has put them into a very tough position.