Because the sentence is "you (pl) will drink tea", not "you will all drink tea". The course allows you all as the plural you, since a lot of people seem to complain when its not accepted in other courses.
You works perfectly fine for most (if not all) instances of the plural you. If you don't normally use you all then don't use it here, and if you find a sentence penalises you for not using it, report the sentence.
I am from New England, and "you all will drink tea" sounds very southeastern to me. We usually wouldn't use "all" at all, but if we did, we would say, "You will all drink tea" (or "All of you will drink tea").
Maybe the course developers think we can't split infinitives. Somehow people have confused English and Latin grammar.
The "will" is split from the bare infinitive "drink" by the word "all" in the comment above about non-Southeastern US English.
I see what you mean, although I thought people only get upset about split to-infinitives. I agree that it is actually perfectly fine to split infinitives in English.
UUUUUUUUUU Why "you all drink tea" I know it is plural but never heard such translation. In ENglish plural sounds same as single in this case so "you will drink tea" should be accepted. The most frustrating is the fact that in some cases both forms are accepted by the system, in some cases only one of them, not always the same.