Why in the world do we have to wait until level 11 to access custom runes?

I know it seems weird, but simply having the option to customize rune pages can make the new player experience worse. Some players will feel the need to optimize their runes right away and are faced with too much information at once. They can then either try to work their way through that information, even though they don't have the game knowledge necessary, or give up on customizing runes and maybe refrain from touching them again later.