It tends to be a fabrication of people miscalculating. You look at the total income sum "Wow!, 120k is twice 60k!" and you judge based on that.
It's similar to items appearing cheaper in the US because they do not include taxes, salaries "the sum" does not accurately reflect the quality of life. The number is higher but it doesn't reflect purchasing power accurately.
It's hard to explain, but I did these calculations a whole bunch of times because I keep considering moving to the US, but ultimately it comes down to:
A) Having a huge benefits package (which is not optional for an employer to avoid paying out for in the EU, especially Scandinavian countries)-- this includes insurances for loss of employment or sickness, but also pension contributions which are actually illegal to avoid in most European countries (5% matching contribution being pretty standard).
B) QoL differences up to and including healthcare, paid vacation time, paid sickness leave and paid parental leave. (from 2w-480d depending on country)
C) Childcare.
D) Taxes (and an accountant's time to file them on your behalf, this is assumed to be a minor cost)
Ultimately I did the math, and I'd have to dig it out again, but unless you're 20-30, very healthy, childless and a low-risk taker that enjoys driving: it is unlikely that you'll be better off working in IT in the USA.
Obviously there's 10x developers who would out-earn me there though.
If you want to earn large sums of money in a European country, that can still be done in Ireland or Switzerland.
FWIW: in my calculations 120k USD in Los Angeles was roughly equivalent to 55k EUR in Stockholm (50k SEK/mo)