For tech, it's largely a different set of reasons, like high wages, no real grievances per se, and the ease of transferring to other companies, plus the work is all virtual so there is no reason why companies cannot outsource to another area where the union has no power, if the workers are just on their computers for work anyway. This latter reason is actually exactly why Netflix is investing heavily in South Korean productions.
As for what solutions union provide - I have already explained. They exist to protect workers' interests. They do not exist to solve company's problems. That is the job of the company.
You said "It is to make sure that the company has done its absolute best to find another solution to the problem," and I am asking, what solutions have unions "made sure" that the companies have done instead of layoffs? I am looking for concrete examples as I often hear this claim but I'm not sure what a union can do if the solution is layoffs, so my question is, what other solutions are there?
My naive view: in the US, unions are all about creating another set of assholes to counterbalance the existing assholes. In the EU, there's at least some thought towards "hey, maybe we shouldn't all be assholes?" (Or at least, not all of the time.)
That doesn't address your question of why it doesn't happen in the tech sector, but perhaps my anecdotal opinion is widespread enough to be added to your (already good) set of hypotheses?
As I said for tech workers it's a different set of reasons.
In Canada, unions are often shop-wide with no mechanism to opt out, which makes them very sticky and allows them to grow predatory if they can maintain enough corruption or apathy. I'm led to believe that many US states have similar problems, but that's only based on how American unions are portrayed in news and fiction.