because that is affirmative order. In English we use the inverted order to make questions or we include do/does.
In all Germanic languages, the word order in questions is normally inverted, and pretty much in the same way. And in all Germanic languages, it is possible not to do give the question a special sense that is also pretty much the same in all Germanic languages. Therefore, when translating a question from one Germanic language to another, you should normally use inverted word order in the target language if and only the sentence is inverted in the source language.
Is a boy eating the lemon? - There is a lemon. I don't know if there is a boy eating it. Please tell me whether it is the case or not.
A boy is eating the lemon? - There is a lemon. Someone - possibly you - just told me that there is a boy eating it. What a surprise! Are you sure it is true?
Do you have something to declare? - I don't know if you have something to declare. Please tell me whether it is the case or not.
You have something to declare? - (1) Someone - possibly you - just said that you have something to declare. (a) Why didn't you say that before when I asked you? (b) How is that possible? You have never had anything to declare before, darling, and I really can't imagine what it could be. (2) I'm a rude customs officer. By not inverting the question I am telling you how annoyed I am that I have to deal with you rather than sit at home and watch tv. You are going to make me work either way. If you say yes, there will be paperwork to do. If you say no, I may feel obliged to search your luggage very thoroughly in order to show how annoyed I am.