We tend to think like this, unconsciously if not outright: "I'm smarter than the next guy, so in a dog-eat-dog system I'll come out ahead. Organizing with a bunch of less-smart people would only hold me back."
Plus, at the risk of too much head-shrinking, I've never gotten the impression that tech workers liked each other very much. There's a lot of disdain in the industry, for the guy who uses that language or framework or operating system that I think sucks. You don't see that so much with, say, truckers. There may be some good-natured rivalry based on truck brands or long-haul versus short-haul, but not the real disdain you see in tech.
I don't think that's it. I'm pretty damn red on economics, and I'm a software developer, but I'm not in a union because the unions I could join are not very good. Either they're more interested in selling me credit cards, or they're for policies which are disastrously bad for a software developer (e.g, paying into a pensions scheme which pays out nothing if you haven't been with the same employer for 7 years before retirement), or more commonly, both.
I could join and try to influence them, but if I was good at that sort of thing, I probably wouldn't have been a software developer in the first place.
Plus, at the risk of too much head-shrinking, I've never gotten the impression that tech workers liked each other very much. There's a lot of disdain in the industry, for the guy who uses that language or framework or operating system that I think sucks. You don't see that so much with, say, truckers. There may be some good-natured rivalry based on truck brands or long-haul versus short-haul, but not the real disdain you see in tech.