It also accepts "What are you eating?" for this sentence. What it doesn't accept is "What have you eaten?"/"What did you eat?" 你吃了什么？ because it's missing 了le, the completed action marker.
Completed actions are not to be confused with past tense.
Chinese does not need to have tenses, they are either guessed ir you need to add another word like yet or later. You need to specifies if it is a completed action or an action in progress at the time you are talking about