As long as the tech job market is good enough that you can find a job when you want to (especially if you have worked for one of the FAANGs), most engineers don't see any need for unions. If they don't like things (including pay), they'll quit. They'll go somewhere else. They'll be paid really well. They'll be fine. What's a union going to do for them?
This is especially true because they're above average as an engineer. It's like Lake Wobegon - everyone's above average. They're sure of it. "Those less skilled may need a union, but I don't."
A union won't happen until enough engineers get blatantly mistreated by Big Tech that the rest think they need protection.