But it does add a factor of randomness that you want to avoid in a tournament.
Not to mention that people talk about "items on" or "items off," but there are far more settings than that. When I play with friends (in Melee...I haven't played Brawl yet), I typically play with items on very low and about half of them turned off. Pretty much everyone I've ever known, no matter what level they play at, turns off starman, heart container, and maxim tomato. I also usually have hammer, the mushrooms, cloaking device, and some other items off. I'll change it up every few months. I'll even turn items off completely for a while. This has to do with how I enjoy playing the game, not with an obsession with fairness. I can beat the shit out of my friends regardless of settings.
Same goes for stages. I turn about half of 'em off and do random every time (or occasionally just select Final Destination). I've never played anyone who leaves Icicle Mountain on random select.
Even though I usually play with some items on and on a variety of stages, "items off, Final Destination" is still a fun way to play the game. Randomness does not always equal fun.