If you're comparing to European jobs, one factor is that the U.S. has higher income inequality then many other countries, which means generally the highly paid jobs pay more, and the poorly paid jobs pay less. This will make U.S. salaries look good in some areas, and bad in others. (The median U.S. salary is also a bit higher than most other countries, but not by enough to explain the differences seen specifically in tech salaries.)

I think there is also not as much of a general tech talent shortage in Europe, so it's not one of the most in-demand jobs outside of specialized areas (if you're a top deep-learning expert right now, yes, but not for general programming / SE jobs). Many European countries traditionally have very strong STEM education, in a number of cases actually overproducing highly skilled graduates in the field relative to the local industry's needs (which is why you see a lot of Spanish, Romanian, Greek, Italian, etc. STEM graduates working in other countries). Not quite as badly oversupplied in the "T" part of STEM as in the "M" part, but still, not a shortage.

edit: Oh, another factor, for right now, that I should've mentioned is that the U.S. dollar is much stronger against the Euro and UK pound than it has been historically (even compared to a year or two ago), while salaries don't respond that quickly to currency movements. There are still significant differences if you use circa 2015 exchange rates, but smaller.