Companies fight tooth and nail against unions because it costs them money. They fight tooth and nail against everything that costs them money. Its just how it works. Someone doesnt want to accept a smaller bonus, screw the workers.
You are right though, healthcare tied to employment IS the only reason I remain at my current employer... (Who pays 90% of my $300 deductible PPO) The pay is marginal and I could make more, but I really like the current insurance setup. That and the PTO is generous..
I don't think unionizing is going to matter. The lower level jobs are being phased out. Once upon a time there were sysadmins and QA testers. You needed people to run your exchange server and people to test for bugs..Now. much of the testing is automated and who runs their own Exchange server anymore? Not us.
I wish I could say, unionize and things will be better, but I doubt it. The only thing that saves me is I have a lot of experience in the field I work in and someone who shows up on a visa or from an agency is never going to have that. If you are easy to replace, companies will do it, because back to my first paragraph, it's all about money.
(side note: I migrated to WFH pre-COVID and I didn't really care if it ended my employment, I'd had enough.. They have replaced me with ~4 people in the office. It would have been much cheaper to throw an extra 20k my way, but instead they now spend an extra 200k and still pay me!)