Because character creation and leveling mechanics aren't about, or need not be about, simulating all possibilities in a setting. A character with fireball who isn't fifth level may well be perfectly in keeping with a setting, something the GM ought to be allowed to have happen, but it could also be something that would be overpowering for a player character to be able to do. There are tons of things that will be possible in a setting I am not going to want to make part of basic character creation, no?