Why cant the teleological argument be applied to God?

What you point out is absolutely correct. We need to presuppose some ultimate cause (G-d, the big bang, etc...) of everything in the universe, by means of which to explain everything else. But now that cause stands in need of explanation and it's turtles all the way down. We can't (and shouldn't try) to justify the basis of any philosophical system; we should rather ask: is the system explanatory of the way things are? does it make accurate predictions about future events? is it consistent with our other beliefs? does it take too many first principles, or only a couple? (i.e. the theory that takes every true statement about the world to be a first principle, incapable of justification would explain the world, but it wouldn't be a very good theory...). But this is the best we can do - we can't find an unproblematic first cause of everything, because then we'd need to explain this cause.