> That's just the way it is, for essentially all species. It has to be, otherwise populations would explode in numbers until it's standing room only for the entire surface planet.
Human population has been increasing for at least thousands of years. Our best estimates put the human population 30000 years ago to about 8 million people, at about 1 ad the population is estimated at about 200 million people, in the 17 hundreds the population is estimated at about 600 million.
That is not a stable population, that's a growing population. You also have to take into account pandemics, widespread violence, etc. The black plague killed 1/3 of the population of Europe, you can be pretty sure that the reproduction rate was above 2 both before and after the pandemic. Millions of people died when Europeans colonized the Americas again the population growth rate would have to be much larger after the event for the population to bounce back.
Additionally adult mortality before procreation does not factor in child rearing behavior. Because why would it?
And lastly all that is pretty much irrelevant especially the habits of prehistoric people, because the change in how Americans raise their children happened in the last 20 years. Not in the 1900s when child mortality went way down.
The thing that changed between the 80s and now was not the acceptability of losing children, what changed was how Americans in particular assess risk, the Satanic Panic of the 80s, the disappearance of Madeleine McCann in 2007, the revelation of how widespread childhood sexual abuse is (before the 90s it was estimated at about 1% of the population turns out it's over 16%), all of these factors made the American public incredibly afraid about their children safety.
It's not that people in the 70s and 60s were okay with harm coming to their children it's that they didn't believe harm would come to their children when they were with other children alone running around. Which by the way was mostly a correct assessment.
In Japan the fertility rate is 1.2 per woman and yet 6 and 7 year olds got to school on their own. Because the culture there believes that is a safe practice, mostly because it is.
Go watch Old Enough, obviously it's a television show exaggerated and not how life really works in Japan, the production team and the parents are essentially watching over the children, but it clearly demonstrates that the culture there expects children to roam the cities safely.
Oh and by the way on average preindustrial families had 5 to 7 children not 10 so 50% survival rate is 2.5 to 3.5. Which taking into account occasional widespread population collapses fits much better with the observed long term growth.