I suspect it's just that when the going is good, people don't really think they need a union. If the prophecies come true and AI starts writing code for us, you can bet people will suddenly be interested in unionisation.
The more I think about it, software engineering is almost completely antithetical to the idea of unions. Traditional engineers have so much structure around their fields because the laws of physics govern a large part of their craft. Not so with SE.
Professional classical musicians require a great deal more training (and talent) than programmers to get to the level where they can make a living doing it.
Mostly because our working conditions are not bad, we are more or less paid fairly, there are a lot of options to move around.
Also it'd be bad if we had other people negotiating on our behalf, for what we probably feel is less pay than we are worth, and the concept of seniority would ruffle and stop a lot of newer grads from getting higher paying jobs over people that have been working already for 25 years in the industry.
That's not a required part of a union. I think one of the misconceptions people have is that if you have a union it looks and operates exactly like one on a Ford factory floor. If you formed a union it could do exactly what you want it to do, and not do the things you don't want it to do. If you wanted a union to do nothing other than negotiate payment for overtime and a new Macbook every year you could just do that.
I've seen the types of developers that thrive in political climates. They manage to get paid way too much money to make poor decisions about subjects they read the wikipedia article on just before the meeting. I honestly believe these are the types that will rise through the union ranks and will use their power for harm and personal enrichment.
Union interviews will be all be standardized and will consist of: drawing UML diagrams, naming 24 design patterns in 30 seconds, and white-boarding a blockchain Rubik's CryptoCube solver implementation.
Points will be deducted if the solution is not an abstract OOP design that can extend to n-dimensional Rubik's CryptoCubes.
So, like today's retarded interviewing environment for most SV companies?
Some possibilities I can think of are: better pay, less work hours or paid overtime, better benefits, more vacation time, minimum office size / screen size / cubicle wall height, a cap on the ratio of employee pay to CEO pay, a right to say no to work that violates a professional code of conduct without risk of reprisal, access to training material, pay transparency, prohibitions against overly-broad intellectual property assignments and a right to work on non-competing projects in one's free time, prohibitions against anti-compete clauses, and mandatory prior notification of layoffs.
I imagine most people who work in tech would have a different list, but there would be some overlap.
Software developers and people in IT are probably two orders of magnitude more likely to be libertarian / anarcho-capitalist and conservative in economic views than your average union worker in another field. That's definitely a large impact as well.
I'm not trying to argue that unions are wholly bad either, but there's a trade-off to be had here. It's not as simple as "people dislike unions because they're too dumb to realize that they want one".