The problem is, not all spells will be built ideally. Here's a bit of an extreme example:
Let's say this spell is available
Spell01
Fire Damage on Target
Weakness to Lightning on Target
If spellcrafting were available, I could buy the spell, delete it, and make this:
Lightning Damage on Target
Weakness to Lightning on Target
Spellcrafting also allows for immersion of identity. For my Paladin, I would make:
Holy Light
Absorb Health on Target
Turn Undead on Target
Light on Target
But if they don't include it, that's fine. That's what mods are for