It must have happened enough that it became the Marine Corps way to make sure that there was no trace of anything remaining in the barrels. And once it was the Marine Corps way, it will be thus forevermore.
Never mind that the stocks are plastic, the barrel is made of a stainless alloy, and the ammo is non-corrosive. Never mind that the armory is air conditioned and humidity controlled. Our memory stretches back to rifle racks in open air barracks on Corregidor (and before). That's my theory on the why of how Marines clean their rifles.
That institutional memory can serve a purpose. If you're shooting old surplus ammo, anything from before 1955 or so, or Soviet Bloc ammo up into the
You still don't have to wear out the barrel to get it clean, but that's my guess where this all started.