You cannot compare being an employer in the US to being an independent contractor with one employee in Germany. Things are very, VERY different here in terms of insurances and retirement. To give you just one example, the usual figure thrown around by workers in the US is to have at least 1 or 2 million for retirement. This is a figure absolutely no regular European employee will ever lay aside in all their working years unless they have a 1%er position.
90k Euros a year even as a contractor and after taxes and insurances translates to netting roughly 40k-50k Euros in a country where the estimated net salary is 2k a month and many, many people have to make do with significantly less than that. I would say average rent is somewhere between 500 and 800 a month not accounting for utilities.
Programmer median salary is 42k a year, senior developer median is 55k. Employees also give up almost half their pay for insurances, taxes etc.
You do the math. 90k a year is pretty great in Germany and definitely in the top 10% or 5%.