Why haven't any of the Game Grumps crew or Pewdiepie been on the Co-optional podcast yet?

I feel this might have been asked before, but why haven't any of the grumps or pewdiepie been on the Co-optional podcast yet?

I don't know about Totalbiscuit, but both Jesse and Dodger knows or have met several of the grump members and Felix (pewdiepie). So it seems natural that at least one would be on the show one time.