The survivors are indeed fit and healthy. The rest dies (often by someone else's hand) and isn't around to be observed.
often ?
Is that a first person observation, guesswork, or dubious inference on the basis of sparse evidence of occasional battles in some parts of the world?
The certitiude of comments such as yours is always intriguing.
For reference, in the region in question there's no archealogical evidence of any pitched battles in the Tanimi and genetic evidence supports people arriving in various parts of Australia and pretty much staying where they got to tens of thousands of years back until the 1800's at least.
Aboriginal mitogenomes reveal 50,000 years of regionalism in Australia https://www.nature.com/articles/nature21416
https://www.abc.net.au/news/science/2016-09-22/world-first-s...
There is ample evidence for homicide among hunters and gatherers, though. We don't like to hear it, because the Noble Savage myth is still going strong.
https://evolutionistx.wordpress.com/2015/06/08/no-hunter-gat...
Yeah, we don't have a comprehensive record of every tribe ever. But violence is attested pretty well among those that we have.