NiimuraJun
I've got a better theory.
I think there are no air benders in pro bending because.... get this....
There's four of them alive.
I think their names are Tenzin, Meelo, Jinora, and Ikki.
If there are any more, we don't know about them, but I'm sure that they would've been brought up by now.
The first logical answer here.
There are no airebenders in Probending because there aren't any airbenders!
The only airebenders are an old guy, his three young (eventually four) children (the oldest is probably only 10 years old), and the avatar who is already playing as a waterbender.
Besides, the game was created back when there was only 1 or 2 airbenders, even if there were enough, there aren't any rules to accommodate them.