Is there Lore behind the Armoury system?

[Niwashi]

Is there some sort of basic lore that explains how a meatheaded tank can just grab a staff and instantly be able to throw fireballs everywhere? Or a geeky bookworm mage can just grab an Axe and turn into a rugged warrior?

Well, it's not like he can instantly become skilled at a new class. It's pretty much like learning a new skill in real life. If your character wants to learn a new skill of a different type than he's learned before, he has to start by getting some instruction and practicing the basics. That's essentially what a level 1 character is, someone who's just starting to learn the most basic use of a new weapon or tool. And it's why you have to start each class at level 1.