I've wondered this since day one
why are forum games not allowed?

Because spam.

Simple it's because they always end up spammy.

Pretty much this. A lot of forum games rely on one word posts or nonsense, such as "counting" games. These serve no purpose for meaningful discussion apart from raising one's post count.

I think we should get some mod approved forum games, because it'd be nice to explore some games with these classy forumites besides Werewolf.