Because dungeons in this game are intended to be difficult tests of skill -even though in practice they’re just superfast meat sponge NPCs twoshotting you with super sayian teleporting speed-. As such, they do not start until level 30, for that is when you unlock the full range of your character’s abilities -save traits-. Plus, level 30 is about the point where most people have a grasp on how the heck to use their class.