Spirits are all-powerful

Absolutely not. Dying and moving into spirit form does not mean you've just won some kind of universal power contest. If the spirit did not have the power while in living form it will not have it in death. Yes, spirits do come into some abilities like moving between the physical & spiritual planes but if they were not magically-inclined in life they are not going to know or be able to practice magic just because they came into spirit form. That goes for any ability or power. A Harpy, for instance, is not going to suddenly become and emblem of Good Luck & Attracting Positive Light because they are now a spirit, just as a Leprechaun is not going to become a powerful sorcerer in spirit form. Spirits do not come into cosmic powers they did not have in life just because they died.