Because "are there/there are" is a construct that takes a different form in Turkish. I am not a native speaker of Turkish, but if I've grasped what I've learned thus far, this is how you would write the sentence you wrote:
Why is this "Why the sandwiches are in the sea? " not accepted as an answer? The position of the "are" is also correct in this sentence and give the exact same meaning as Duolingo's correct answer, which is "Why are the sandwiches in the sea?".
One of the benefits of learning another language is that you also end up understanding your own better. I'm finding that Turkish is really challenging me to analyse English in order to understand the Turkish grammatical structure.