What gives him the right to do that? Creating billions of sentient life forms and then not giving us a purpose beyond worshipping him is a) egomaniacal, and b) irresponsible. If you answer the question by saying "God is God, it can do anything it wants" then a) you're not answering the question, and b) you're not allowing yourself to answer the question.
You've given no reason to accept the notion that it would be "egomaniacal" and "irresponsible." You've merely asserted that to be the case. How do you know that would be case?
You can't be getting this from atheological ethics, because in atheism we're all just blobs of protoplasm without any rights whatsoever. You can't very well set yourself in a position to criticize (Christian) theism while at the same time lacking a warrant for your own beliefs. Unless you believe in moral absolutes, you are in no position to render a value-judgment on Christian theology or ethics or that of any other religion, and if you believe in moral absolutes, you need to provide a warrant for those absolutes. In secular ethics, however, you don't get moral absolutes, you get,
"What was good for our ancestors may not be good today."