This is not true. There are many such opportunities if you ask, especially now.
> it's not that you will get a 20% pay rise every time you switch
If you work in IT (generous, but it is HN...), this should be your experience unless you switch more than once every two years. Then everyone will mistrust you, but you can still do it as a contractor.
> On top, it is extremely hard to find software development jobs that pay around or above 100K. Most jobs are around 60-80K.
Making around or above 100K in the EU is indeed very unusual. Of course, such numbers mostly make sense in the US because it is (socially, in terms of security) a desert hellscape. The lower top salary in the EU comes with the benefit of knowing that if you go blind you won't have to die shitting yourself in some crackhouse.