it would make it easier to survive in the tbm plus people would use it more for cv based missions
The TBM can only survive by maneuvering. The gunners are just too limited and do not have enough firepower to make a difference. Attacking a formation of TBMs means 3 easy kills or two free kills and one maneuvering target. An unloaded TBM can surprise quite a few fighters with its 2*0.5 forward guns.
Having said that, there is no real reason that the TBM should not get the formation option, except for the technical difficulty of making formation T/O & land on CVs. If there is no easy solution to that issue, then it is not worth the effort to implement for just two very lightly used planes.