I think this is because in English, "coffee" is not counted. That is, you cannot say "a coffee" in English because it is not a definite quantity. You can say "a cup of coffee" instead. This is one of the instances where translation between Arabic and English do not exactly match (specially in the definition of some words).
Another example, you might say in English: I like nature (without defining nature because it is a general term and you are talking in a general sense). But in Arabic, it doesn't work that way. You would say it as أنا أحب الطبيعة, where الطبيعة is (the nature); It comes defined with "AL" and it would make no sense for an Arabic listener to hear it in this sentence without AL.
thanks, it is the first time that I try to learn a language on the base of another foreign language