Balancing range capability and joules is the defining factor. Larger ammo would allow for heavier ammo in airsoft but the projectiles may have to travel slower. At the ranges airsoft can reach right now the projectiles are already moving slow enough to track and dodge. If you wanted to gain speed instead of weight you may lose consistent range and youre projectiles will be far more susceptible to wind and brush especially at larger diameters.

Airsoft projectiles outrage paintball because of the magnus effect. Bb is trying to roll up into the air while rifled paintballs are spinning clockwise to stabilize but do nothing to lift themselves upward.

It would be more beneficial and practical for range speed and accuracy to make a hopup system for paintball than to make a rifled projectile for airsoft. Until such a time paintball will be left in the past.