It's not illogical at all. It just means that the characters who get trained as demon hunters are already experienced fighters. They don't have to be, but there's nothing illogical about it. However it would feel very anticlimatic if you have this great starting experience where you learn how to be a demon hunter, you get powerful glaives and fight powerful demons, slowly become a demon yourself and then you go and fight quillboar in the barrens.