Well, we're about to take a 160-175 year time skip, so issues will start resolving themselves xP

Also, Arguis is a god, he can read. Mirtagn is thousands, if not more, years old, so that explains him. Zerabil was born a reader, and Uri are the creations of Arguis, with the ability to pretty much suck knowledge from the very land during their birth. I only assumed Garakains read because they've got a city and something of an aristocracy.