Forum games totally work! They do it over on BBG, all you need is a middle man. The two players send their cards to that person and when both have done so, that person post them.
Edit: Or did you mean that GtG would probably get annoyed if their forums started getting a lot of post of people playing another game?
No, he was asking WHY a forum game wouldn't work. Imagine a ? at the end there.
I've been totally enjoying forum games! It's a very different experience, leading to much more thinking about individual moves. You don't always need a middleman if you trust each other and use spoilertext.
FYI: Checked with The-Powers-That-Be, and they are fine with folks doing PBF games of games they didn't create / aren't involved with, so long as those threads are in the Off Topic forum. 