It is called Pythagoras's Theorem precisely because Pythagoras, according to legend, first proved the statement of the theorem within the framework of Greek geometry. Proof matters to mathematicians, and Pythagoras is justly famous for his achievement. There is archaeological evidence that much older civilisations than the Greeks, such as the Sumerians, Babylonians and the Egyptians, were persuaded of the correctness of Pythagoras's Theorem. But there is no surviving evidence that anyone other than the Greeks produced a satisfactory proof, nor did any other civilisation even possess a formal mathematical system capable of representing a logically correct proof.
Very interesting! Still, I don't know if the Babylonians developed formal proofs in their time (the greeks, in turn, did).