Probably because (as far as I know) you can’t interupt reload animation.
So making rifle grenades auto reload (aka force player to reload) could cause situations like: you fired a grenade, you want to fire your rifle and take off your grenade device, you can’t because you are in reloading animation, enemy kills you because you were locked in animation.
So it could cause a completelly artificial problem.
You could say the same about the anti tank rifles yet we automatically reload AT rifles.
You can change from AT rifle to main weapon at eny time. So you aren’t defenceless unlike with rifle grenades.
True! I wan’t auto reload as well for the grenade launcher!
It might do good to balance if you have to make a choice between hand-grenades and rifle-frags.