IMHO it's the culture and the VC environment. There's simply not that much money around to throw at moonshot projects, therefore you don't have many unicorns that lose a few billion euros a year paying extravagant salaries for talent. Obviously EU is a very rich place but people with money invest their money in stuff that are profitable right away, likely to be profitable in short term because a giant company is behind it or invest in the US companies if they are more adventurous.
You are not getting that much in net salary above certain amount.
Europe is a hellhole if you are a young ambitious individual.
How does one identify a well paying non-union employer? Noname companies in almost all cases pay much less than union employers.
All I know is a lot of great engineers at my company are internationally refusing more pay to non union jobs. I.have no idea if it applies to other companies, or what the benifits are