There is a life motto that says
"If something is not broken, don't fix it". Cipsoft actually did a very good job at balancing the 4 vocations, so we think it's much more beneficial for the server to carefully add custom spells and new features to the existing vocations, than to delete them and make new ones.
If we did, the server would probably be anything but balanced. Personally, on every server that I have played that had custom vocations I was able to find one that topped the rest. And mostly also one that was plainly useless.