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The movie established back in 1987 that one could use the power within Castle Grayskull to become a god with a Jack Kirby helmet. So the Sorceress is part of a line of defenders forgoing that power.
Who else could you see conquering the place and becoming a god? Hordak? The Filmation version seems oblivious to the Power of Grayskull, but most other fiction has written him scheming to get in.