Apparently you think that the cull sows are representative of the whole population at those farms.     They are being culled for a reason.

You are also assuming that those "bags of bones" wouldn't have died a few litters ago fighting for food in an open lot. 

You also have no idea of the age of those sows.     Animals get frail as they age just like people do.

Sows get old, they get sick, they get hurt and they get culled.          The sows that are in good condition don't get culled, they get bred.