I would guess that having to only deal with 2 characters and a small arena as is the case in a fighting game would allow for much better graphics as opposed to a game that has larger arenas and more characters on screen at once. Fighting game characters can be made up of more polys due to their being only 2 of them on screen at any one time whereas an adventure game has many more characters to display but most likely the same poly budget. You cant have everything all at once, you have to make concessions in one area to free up space for another.