It’s a balance thing. Most bullets in Overwatch are hitscan, meaning the damage is instantaneous exactly where you are aiming at the moment of the button press. Very few bullets are projectiles, namely Orisa, Roadhog’s secondary, and Doomfist’s knuckle gun.
Projectiles in Overwatch have no falloff. This extends to Genji’s shurikens, Pharah’s rockets, Zenyatta’s orbs, and Hanzo’s arrows. Mei’s icycle is like these.
If you nerf Mei’s secondary I feel you’d have to fairly do so with all projectile damage.
If bullets had no falloff you’d have McCree who could snipe and shotgun heroes would have disgusting damage even at range.