Because I guess whoever put in the correct answers didn't think anyone would combine present simple and present continuous like that.
I can't think of a context in which this sentence would be correct in English. Present simple is meant for repeated actions (e.g. I drink a cup of tea every morning), present continous for action that takes place now (e.g. you are drinking beer while I'm cleaning up).
Okay, I may have come up with a context for this: I drink a glass of wine now and then, you are drinking all the time!
"du trinkst" is the familiar "you"-singular. "Sie trinken" when translated as "You are drinking" would be the formal "you" or used when speaking to a stranger- but still singular. "Ihr trinken", I think is the plural formal for "You, meaning many you's, are drinking". I'm not sure if what I have inserted here is completely correct or not.