Why is going to a sorcerer and seeking help from other than Allah considered shirk (polytheism)?

A Muslim must understand that the greatest evidence for ruling a matter to be an act of disbelief (kufr) is the presence of a sacred text (nass shar'i) concerning it. Seeking help from other than Allah is polytheism (shirk (definition)) because the Prophet, peace be upon him, said: "Indeed, help is not sought from me, but rather help is sought from Allah." And whoever comes to a soothsayer or a diviner and believes what he says has disbelieved in what was revealed to Muhammad, and that is because he associates them with Allah in the knowledge of the unseen. Allah Almighty said: "Say, 'None in the heavens and earth knows the unseen except Allah.'"