Why do major international universities still have courses in languages other than English? As an example ETH Zurich, which is arguably one of the best universities in Europe, still educates Bachelor's students in German:


  Please note: the Bachelor’s degree programmes begin in German. In the second and third years of the programmes, some of the courses may take place in English.


This doesn't make sense to me as it's obvious that the vast majority of technical literature and textbooks are published in English. And pretty much every major scientific journal only accepts works in the English language. This also decreases the amount of competition between the applicants as German speakers hold an advantage over others (not to say native English speakers don't hold the same advantage back at home, but English is by far more common).

So why didn't every single major university switch to English already?

EDIT: I am mainly referring to programs in the STEM field, such as engineering
ETH and all other universities in Switzerland are funded by the government and offer college education for minimal tuition. While foreign students are welcome to attend, the primary purpose (at least at the undergraduate level) is to provide education to the locals. Nonetheless, that doesn't seem to be a limiting factor, especially at the graduate/research level since ETH has been recently ranked one of the most international institutions by Times Higher Education.

There is a wealth of excellent textbooks in German, and many fields still have original research published in German.
Also, the majority of students don't pursue academic or research careers and thus might very well spend their entire career speaking almost only German.

Plus there are practical considerations too: asking non-natively English speakers to teach in English is most certainly going to decrease the average teaching quality. Now if it's to give lectures to a class that is almost entirely German speaking, it borderlines the absurd.

The question seems to be more: why would some universities switch their teaching language to English?