I think many Europeans can't imagine moving to the US (higher work load, less social safety, higher violence) and employers well know it and can keep salaries depressed.
Look at all the top surgeons and hospitals in the world for everything from neurosurgery to cancer treatment. They're all in the United States. The median level of care might be much better in Europe but they are behind when it comes to cutting edge treatments, rare diseases, and so on.
Out of stubbornness, I fought with the lab for months to bill it correctly, and eventually got them to write it off by emailing the CEO.
And most of the expats I know who did it seemed to be working for SV companies. So they didn't leave the higher salary behind.