The US has a currency that they can print out of thin air and the rest of the world wants it no matter what. Countries need USD to buy commodities and Americans are reaping the benefits. Cost of living in the US I would say is a lot higher but if you are making 300k as an FO I don’t think it’s an issue. The US is one big single market where as European is a fragmented market.
I don’t think Europe will ever catch up but there are now no American pilots working abroad really so it may slowly lead to salaries increasing in the ME and that has an effect in Europe.