My Italian girlfriend agrees.
I think it doesn't translate directly, direct translation would be:
"Why do you eat much"
But as you say "so much" is always said "cosi tanto", so it's difficult, I think "Why do you eat a lot" and "Why do you eat so much" should both be accepted.
adverbs modify, clarify, qualify or quantify verbs, adjectives or other adverbs. they are invariable, meaning they do not change based on gender or number. but be careful because some adjectives are also adverbs (such as piano, which as an adjective can mean flat, but as an adverb means softly). so, be sure to change the ending when you are using one of these words as an adjective, but not when you are using it as an adverb.
"YOU ALL" should always be an accepted translation for 2nd person plural translations into English. There is no reason "Why do you all eat so much" should be marked incorrect. It is the only way to practice/differentiate between 2nd person singular and 2nd person plural and saying "you all" is very common in American English.'
Am I missing something?