More-so addressing the issue at hand then discussing it, I'd probably have to say there's two main reasons why so many series seem to use such default weapons. Well, one really...
Because bows and swords are mainstream, there isn't exactly a lot of research that needs to go into it, therefor artists for comics/animation, don't need to study all that much and can just take a basic shape, and add a bit of flare to it, and boom, you've got yourself a 'fantasy weapon.' Same thing as with prop studios for live-action stuff.
On the other hand, more 'work' would go into making something like a Chakram, or a morning-star. Due to that, it would take more time, and thus cost more to get that drawn/animated, or made by the prop department. So, time is really the biggest factor.
Time, and swords/bows are just far too mainstream.