Prospero does not like being reminded of his baser side, that all humans have, and that is why he pursues the goodness of Ariel instead. It would have been even more risky for Shakespeare if he had written a play that had Prospero fully embracing crazy black magic and he also does not even attempt to write about magical spells or incantations.
Prospero uses his great intelligence to win himself greater power. It is also to be noted that Prospero, mighty magician though he be, has no power to bring two young hearts to beat as one.
If he had introduced the forms and ceremonies of conjuration and adjuration described by Scot, the effect would have been either ludicrous or disgusting. Prospero has to choose between magic and his Dukedom. He is unpleasant with Ferdinand and his daughter.
It is quite unnecessary to say, could never descend to the base devices of those who, having gained a measure of superhuman power by a compact with the great adversary of souls, became the ministers of his dark purposes.
Shakespeare wanted a happy ending and in order for this to happen the characters and events must be manipulated through magic.
Carbondale: Southern Illinois University Press, And make thy weapon drop.