Probably because, although very common and useful, y'all and you all are generally stigmatized as not being 'proper' or 'standard' English. Personally, I think if they're accepting British-only sayings (like dear=expensive), then y'all should be accepted too. But they may be trying to translate the Dutch words into a more 'regionless' version of English... In which case, no.
And how is that going?
Learning a language is recognising what you hear and see and translating it to you own language as well as the other way around.
What good is learning a language if you don't know what they are saying? If you don't know what they are saying you haven't accomplished learning the language.