Its actually funny you say that, since the game was originally designed to have that very thing happen. I assume they decided not to go with that because players wouldn't be able to level up because all of the mobs would be killing each other. Not only were you competing against other players but the mobs themselves lol. The monster correlation chart was pretty much the original plan of what mobs would fight and eat what. I think they still used some of the system every now and then with things like the NMs you have to feed other mobs to spawn like that slime or antlion in abyssea.
So while you can say it doesn't make sense to do it because we have never seen SE use it, the game technically was designed to do it in the first place.