It is not a normal English sentence. For example your sentence should be better: "Why is "where the girl eat" not ok?". The finite verb must be in front or driectly after the question word. Here it is either: Where does the girl eat? or Where is the girl eating?
I hope I could be of help.
"Where does the girl eat" is the correct choice. I'm a native American English speaker but not a grammar expert. I will try to explain.
There are two verbs in the sentence, "DOES" is a helping verb and "EAT" is the main verb.
"She does eat." - This sentence tense is present continuous. It is more commonly taught as, "She is eating."
"She eats." - This sentence tense is present simple. There is no helping verb.