Isn't it necessary to put "are" in "where you at"?
Yes, technically it is necessary to put "are" in this sentence. The reason why "are" is left out of this sentence is because they are speaking very informally and not grammatically correct. It is very common in spoken English to leave out filler words like "are" to shorten the sentence when speaking. However, it is not common to speak in this way to someone you don't know well or in written English unless you are texting.