Unions do things like engage in age discrimination by favoring senior members over all else.
They do things like throw up oppressive barriers to entry, and limited licenses, in order to F over everyone who is not already in the club.
Unions are only a good idea for exceptional 1 size fits all situations.
For example, there are only a couple major airlines in the industry, and if you get fired or leave, your career is permanently messed up, even though you spent a decade training to be a pilot and getting the necessary flight hours.
In this kind of situation, where an airline has excessive bargaining power, and workers have no other option, a pilots union makes sense. Even though a pilot job is a high skilled position.
But for tech? No way. Engaging in collective bargaining to get the small amount of negotiation power just isn't worth the drawbacks of a 1 size fits all solution.