Lack of time,
Too overpowered in Oblivion,
Laziness,
Choosing to make the game simpler.
One or more of those things.
I don't get it, I can understand why some people think alchemy+enchanting+smithing is op, but not Spell making. How can it is OP when you decide the potency of the spell?
And even if you create a 100 damage for 3 second fire ball it cost massive magicka an altmer with mage birthsign with 100 intelligence can only have 300 or so magicka in Oblivion while a 300 damage fire ball cost around 200 magicka.
And even then at level 25 it does not one hit a gloom wraith.