When comparing European and USian salaries, it is important to remember that companies primarily care about what it costs to employ you, which is different than the number on your paycheck. Generally speaking, the cost to employ software engineers in the EU is much higher than in the US, so even though the differences in salaries seem large the cost to the companies are often similar. The proportion of that cost that goes to salary is simply much higher in the US.
Places like Silicon Valley and Seattle are still outliers but if you looked at "typical" software engineering salaries in the US the differences in salaries are explainable almost entirely by differences in payroll costs (various taxes, regulations, etc) that the employee never sees.
Do you have evidence of this? This web page suggests that the extra costs in the UK are not that high: https://www.crunch.co.uk/knowledge/employment/how-much-does-... In some European countries it is hard to fire workers, which could be a serious issue, but in general it is hard to believe that hiring in Western Europe is not cheaper than in the US, usually.
I work at a global (nominally European) company with some of its largest offices in Europe. When I have floated the idea of hiring in Europe instead of Seattle, because of wage costs, I have been informed that it doesn’t save much money despite the wage difference. The employee overhead in the EU is (to my USian mind) astonishingly high. The UK is not as bad as the continent, but I floated that too and was informed it all washes out the same.
In short, the apparent arbitrage opportunity doesn’t actually exist. As you would expect in any semi-liquid, semi-efficient market.
Typical salary for a senior dev in Berlin is 65k. That's about 73k fully loaded cost for the employer, IIRC. Nowhere near these numbers (which apparently are still way too low for senior devs in SV).
Places like Silicon Valley and Seattle are still outliers but if you looked at "typical" software engineering salaries in the US the differences in salaries are explainable almost entirely by differences in payroll costs (various taxes, regulations, etc) that the employee never sees.