Because overpenetration doesnt stop the enemy.
Well, there goes my favorite M829 APFSDS-T 120mm rounds...
In Desert Storm, the M829A1 “Silver Bullet” was known to penetrate a five foot thick sand berm, through the frontal armor of a T-72, through the engine block, and exit out the rear of the tank.
But that is a wee bit bigger than the 5.56mm M855 NATO ball round.