Major US tech companies earn a profit-per-employee of $200-400k[0]. That's across every employee, so depending on how you attribute value the profit-per-SWE is much higher. That gives a lot of headroom to drive up salaries when competing for hires, and in part sets the market price. I suspect that low European software salaries are just a function of low profitability of European software companies. e.g, spotify is (…
The US dominates the software market (mostly for historic reasons but also for its unrivaled entrepreneur/VC/risk-taking management style) which sells it products everywhere in the world, earning much more on that way. And the remaining software products in Europe are acquired one-by-one (e.g. Skype).
Having said all of that: Being a software engineer in Europe is a privilege with a good income situation.