I think a big reason is that there aren't a lot of good writers who are also good illustrators, a lot of writers just don't have illustrations in mind when they're writing a story. I would assume that its also more expensive to print a book with illustrations instead of a book without...
In most fantasy, it seem to me that the strongest human quality is potential/the ability to adapt. It might just be because there's a lot of stereotypical dwarves and elves out there that humans just get to fill in the blanks, but I've noticed a lot of stories that have humans poised to take the...
I don't know to much about Mort, haven't read it yet. If you want a Discworld novel that's light on wizards, I'd recommend Monstrous Regiment. That or Small Gods.