I can think of two reasons

1. Productivity - US is a large English speaking country, the mobility and productivity is great. Having a slight edge over others will make you very valuable, especially considering that many US companies are global.

2. Unionisation & socialism - At least in the Scandinavian countries my observation is that management/govt/unions holds a position that software developers should not be paid a lot more than other employees. And developers accept that, because "that is how it is down here". I pity the condition of highly qualified doctors, who after spending a decade in study/residency get only a small premium above average. And in contrast taxi drivers who started early in life own a lot more in assets.