Answer:
1. Because European programmers havent rallied together to demand more
2. Because if larger companies can offshore to places like Poland and save 20-50% in a long term labor play then they will and when 4-5 large companies employ 95% of the engineers in that city, they can define salary bands and keep them where they want
3. In the US, there is a bit of a disconnect on which jobs pay what, for example a social worker in the us makes like 35k USD per year, if you have a masters maybe 50k, compare that to Switzerland where they make like 75k without a masters. Or a hair stylist, lieklynot making very much money in Switzerland but in the US you can make a ton and they typically charge more in the big cities in the US than they do in European ones. At the end of the day its all supply and demand and what value the culture puts on things.
4. Basic supply and demand - lot more tech types in EU than in the US from what I have seen, larger labor pool drives salaries down
As an American engineer working in Switzerland I see this far too often, people wanting to come here from Germany, Poland, Czech Republic etc as the salaries there are crap, and really cant blame them. Germany for example, highest tax bracket is reached at like 50k and things arent that cheap relatively speaking. If it were me and I lived in any of those countries, I would be starting my own company or moving to a higher paying country immediately and working the majority of my career there, then retire somewhere cheap.