Because the airline will not take possession of firearms or similar items as they would other items.

For example, if TSA discovers a flammable aerosol in your checked luggage that is not allowed, and the airline can not get hold of you, TSA does not keep the item, the airline takes possession of you. The name on the destination take and flight info is recorded, and if the passenger contacts the airline, they can try to have their aerosol shipped to them the proper way.

However, some things they will not take possession of. Often, more so depending upon the time of year, we find stray bullets in bags, no guns, just bullets. Again, TSA does not take possession of the bullet, NOR will the airline. The airport LEO takes it. This is actually pretty common throughout the year - stray bullets. And lets call them unsecured firearms, with no passenger to be had, well, it will not go, the police will take it.

We do find firearms not properly secured. The last one I found was a .45 rolled in news paper. The police went down to the gate, and got the man who's bag it was. But now imagine the situation I am describing where the bag is shipped to the wrong location, the lock is cut because the bag has to be re-screened - and there is no passenger there. The police will be given the gun.