The reasons for lower salaries in my opinion are:
- most developer jobs are in "real" businesses, which see IT as a cost center
- compared to US, there is a vanishingly small number of technically innovative IT-first companies, so there's little competition for talented professionals
- taxation is progressive and harsher in general due to being the primary source of govt income in most (all?) European countries, and increasing gross salary kinda stops making sense at some point
It would be great, if these issues were addressed, and there are well-known ways to do that. But I'm not sure, if increasing salaries is a noble goal per se. While much lower than in US, they are still decent and allow one to live a comfortable life. So it's rather US being an outrageous outlier (no wonder, given that post-WWII economy is operated on their terms). The elephant in the room here (as almost everywhere in the world) is real estate. Now that one (and prevalence of mortgage) is a huge issue, and it needs to be solved at all costs, even if just for the sake of younger people.
As for salaries, most developers simply don't generate that much real value (real, not money). Though they somewhat compensate for inproportionate incomes on a "higher" level of economy by over-spending the surplus above the necessary living costs, though I doubt it's by design.
Overall, I'd argue, that developer (and many other occupations) salaries should be lower, so that we had fewer get-rich-quickers to work and to compete with. To be clear, I'm not saying that a dev should earn as a janitor, but rather on par with other "highly skilled" occupations.