Being able to use a flamethrower vs. a fireball is not customization, it is nothing but an OPTION, predefined by the devs. Customization is when the player can experiment and create their own options, such as spells, and further make the world their own. Hey that enemy i just took down? That was my spell, my creation. Like that.
Another thing is that what if you started as a purist mage, and kept making new spells as you get stronger and stronger (as you should) then your spell book might consist of a hundred variations of a fire spell, 50 frost spells, 80 Shock spells, 20 invisibility spells, 30 charm spells and so on. Eventually it was all just clutter in the spellbook.
Keeping a set number of spells with various effects that they can achieve is quite awesome, imagine how bad this would get to program if you added spellmaking.
Spells should also scale with 1, your level and 2, your perks. By combining the pre-made spells with perks means you do get a little bit of customization with them.