they simply aren't daily bosses but the daily mechanism is the arena itself, u can't kill the same but once-maybe twice per ss then unable to kill it again until a whole rotation have happened so that's also why they are nemesis and aren't listed, there's not any special reason but that they don't belong to the category itself and 5 bosses sharing cooldown could be considerably troublesome (specially the part that you can kill them close to ss).
the only way to actually make it worth it were creating a "drean courts arena" cooldown but i don't see this happening any soon since the feedback about bosses no archfoes (as raxias, arena, etc) with similar cd was forwarded and answered but i dont remember what they said at respect.