Using powers that aren't one's own always comes at a price.
Even deities are no exception. Vishalon, the God of the Liches, had become a deity by accepting gifts from the Lord of the Storm, yet ended up unable to bear it and fled.
Mere mortals, how could they possibly master legendary powers without enduring the passage of many years?
Katherine used her own legendary power to forcibly elevate others, granting half of the Elf Tribe immense strength but also inevitably infecting them with magic addiction.
The power, now merged with their bloodlines, could not be eradicated. It would be unconsciously used in daily life, and then the magic addiction would become more intense, draining everything. If Elves were short-lived, it wouldn't be such a big problem, as with careful management, they would die of old age before the addiction took hold.
But Elves are a long-lived species.
