That's the point. It was designed to wound and not kill. Wounded soldiers require other soldiers to get them off the battlefield and out of the fight. Plus 300 rounds of .223 weighs about the same as 100 rounds of .308 ammo so you can pack more.
After "tumbling", this is the worst poorly defined myth.
The 5.56 round was designed to fragment, EXPLOSIVELY.
There was never any ingtent to "wound".
The problem is the wounded can still fight you. They need to die instead!