While I'm not opposed to the idea conceptually, I'm afraid that the majority of n00bs are unreliable and any pbp game made up of just n00b players will quickly flame out. Here is a recent example.
That game barely lasted more than a month before it was shut down for missing or barely participating players.
It's one thing when you're playing in person or via chat or something. But pbp takes a big time commitment from the GM and the players and as someone who GMs on occasion I don't feel like it'd be worth the effort to construct a game knowing chances are it'll flame out in a matter of weeks.
Heck, some games flame out after a few weeks with veteran players. But the odds are even worse with new players.