Because the thing people refer to when they say “linux” is not actually an operating system. It is a family of operating systems built by different groups that are built mostly the same way from mostly the same components (which, themselves are built by separate groups).
In the same way that Americans speak English.
Sure, their language is mutual intelligible with English, but if an Englishman comes over here and asks for some chips, they’re going to get a bag of crisps. They’ll mess up verb conjunction on a bunch of collective nouns.
And bless the souls of my Australian mates who come here and call everyone a cunt.