my personal headcanon is that the reason that there are only 11 recognized wizarding schools, which of course seems like a ludicrously low number given that 3 of them are in europe alone, is that other styles of magical instruction (individual mentoring, generational covens, etc) are actually very common outside of the UK but are looked down upon and not considered proper by the hogwarts crowd
The simplest solution to justifying the imperialism of the author is to extrapolate it as imperialism in the text. You’ve only got a certain number of approved schools that the snooty European ones give their thumbs up to as acceptable by their prescriptivist standards of magic.