Obviously you never played halo custom games back in the day because you seem to only think of custom games as highly competitive when they were truly in fact ridiculous game modes put together by the community. Usually infected game modes but also vehicle races, monster truck battles, octagon matches, a game called "fat kid", Michael Myers, ghost busters and so many more. Of course you won't know what any of these are like unless you actually played them but my point is that they provided a way to play halo that was not originally intended and therefore made it a fps that was unlike any other at the time.