This is the right question to ask. You can reason your way around things, but occam's razor reigns supreme. Injured people can still do lots of work, as our most important tools were our brains, not our bodies. It's not hard to watch for predators near camp while sitting at the campfire, or to keep an eye on children - even if you can't resolve issues yourself. You could sit around making crafts for the tribe, repairing clothes, and more.

There's just way too much benefit to keeping the injured around. We don't need everyone working at top physical condition... ever.