Collective bargaining is inherently about eliminating competition between workers. That is, it creates a labour monopoly to balance out a labour monopsony. That might (might!) be a good thing for autoworkers. It would be an unmitigated disaster for software engineers, and the tech sector in general.
People who write code are unbelievably productive compared to just about any other job function, and software engineers (who we'll define as people who write code for tech companies) are even more so. Part of eliminating competition between workers is making sure that nobody works harder than anyone else. That would be devastating to the productivity of software engineers, which would make them less valuable to tech companies. That would lead to a decline in salaries, and general stagnation of tech.
Another aspect of preventing competition is seniority. One of the archetypes of the tech world is the wunderkind—the brilliant kid fresh out of school that rockets to the top. A union would tell that kid, "sorry, you'll have to pay your dues as a junior engineer for 5 more years, then you'll be eligible for promotion. In the meantime, we'll give that job to cwp, he's got the required seniority."
Just think how this would affect tech companies. Sorry, you can't pay more to attract the best. If you're really careful about interviewing, you might be able to find some people with both skill and seniority, but it'll be tough because smart, ambitious people have all gone to Wall St or whatever industry has the best opportunities.
