Because science and its method cannot answer "why?" questions. "Why" implies implies intention, such as that of an intelligent designer or a god, and science is not capable of answering such questions. If you were a scientist, you would have known this. It's common knowledge among scientists.
"Why" does not always imply intention. It may simply be a way of asking about causes, which science does all the time. Furthermore, to state point blank that science cannot address matters of "intention" is perhaps your opinion, but little more than that.