Polytheism By Mistake
Short answer is, because there is no argument for shirk. Only way we would know if Allah has a partner or not if He Himself told us so. If one is to do serious research, the authentic sources like the Qur’an, not only is there an absence of polytheism, it straight up argues against Shirk. So you can’t argue for Shirk on your own nor does the only source of knowledge (Allah Himself) that could confirm it, confirms it, instead that source straight up rejects it. Long answer is this video: Watch from 1:45 to 28:35. https://youtu.be/zEO80jAY-bM?t=105