In short: This happens because __in general__ engineering, __on average__, german work is superior. Mind, this is not due to something inherent to germans, but due to our social environment. In germany you tend to get paid more* for your work and have more time to complete your work because it's expected that you deliver quality. Meanwhile outsourced work is paid little and expected quickly, so foreign workers (especially east-european) have little incentive or reason to improve their work quality and even if they care, have little opportunity to do so. (No time to actually read "Perl Best Practices" when some german manager wants the work done 'now'.) Added to that there's also the fact that higher education in germany is for all intents and purposes free, adding just another early education edge for people here.

* higher wages all around, illegal to work without health insurance, law-mandated minimum paid vacation (24 days), all (real) sick days fully paid