Time to pick up it (fuse time is lower because grenade traveled through air to you), aim it, and throw it somewhere is longer than fuse of grenade.
Throwing back grenade is not usable against stick grenade, as german I always wait little time before I throw it to enemy (enemy cant run away due to lack of time), so grenade explode in air sometimes or with 0,5 sec. delay on ground. Every skilled player do it.
I think that Tripwire never implement it. Best way how to throw it back is catching one in air, but you hold your weapon and also grenade is not soft ball.
Grenade as design is weapon to kill your enemy, fuse time is managed to prevent throwing it back by 99%.