Seeing as we have hundreds of different spell effects, I wasn't expected this at all. It made sense in Fable 3 where you had...6 spells, I believe it was? You had about 15 different combined effects to choose from. Now imagine putting animations on all of those, and then try to imagine it when you have 100 spells, each able to combine with any other. Yeah, not easy stuff. Even if it was easy, it would be quite time consuming and wouldn't make much sense anyway. I could understand maybe combining spells from the same school, but combining from different schools would just be strange.
On the bright side for computer users, I'm sure there's someone dedicated enough to make a mod for it one day.
There aren't "hundreds" of spell effects, there are ~85. But you are right, most combinations wouldn't make sense, and they wouldn't have to implement those. Just the once with interesting effects.
I tought the replacement for spellcrafting was the ability to cast a spell in 3 different ways; the ordinary projectile, the continuous one (flamethrower) and the runes.
Not to mention the ability to use 2 spells at once.
We always had 3 different ways (on touch, on target, on self), so that's hardly a replacement for anything.