We have charts that track 20 levels. Why not for spells?
Because manage spell slots for 20 different spell levels would become a nightmare...
3.5e warlock invocations are divided in Least, Lesser, Greater and Dark. Just give a name for each spell tier/level/circle/whatever; Tiered magical progression exists in TONS of games.
And being a mage is far more harder in other games. For eg, on Gothic 2 - returning(not vanilal g2), you don't start as a magician. Wanna be a necromancer? Ask Xardas for apprenticeship, wait until he decides, become a novice of darkness, train your spirit(mana) from expensive trainers and your INT by doing intellectual things like reading books, runemaking, alchemy, learning new languages and when you have high enough INT(which can't be trained like STR/DEX), you talk to Xardas and do the initiation into the circle of darkness. Go kill a novice of fire mages in a monastery full of powerful magicians, find a good way to assassinate the target and after it, you only need to learn the first circle of darkness, purchase the book with reagents requriement from Xardas and pay for him to teach how to make the Arrow of Darkness or Summon skeleton run. Get the reagents(black pearn, skeleton bone or wathever) and make the rune. To cast spear of darkness, you need circle 3 which can only be obtained on ch 3.
Talking about TT game, GURPS magic is not simple either. For eg, GURPS is not a game where you learn how to cast fireball. You first learn how to ignite fire, then create fire, then shape fire and only after you can create and shape the fire, you can learn how to make a fireball. All things that you can make with shape fire alone are near impossible to translate into a CRPG. Magical aptitude and magic costs is 100% tied to the DM world "settings", if is a no magical setting, you can't learn magic. If is a high magical setting, magic aptitude is easy to obtain.
