> Then please explain why birth rates throughout human history, when life was vastly more difficult and dangerous than it is now, were so much higher?
Because having kids then was a way to increase quality of life. The kids could be put to work from a young age and help make money. Now, with so much modern tech doing physical tasks efficiently, a kid isn't going to add much value and instead is going to be a money sink.
The past is a foreign country. I do not believe "quality of life" in economic terms is a sufficient explanation. The simple Darwinian fact is that, if your culture did not value reproduction sufficiently, through whatever means, it would not have survived to the present. Remember, infanticide was widely practiced historically, and while not as convenient as birth control, provides a significant fraction of the same "benefit".
I do find it a bit ironic that "free child slave labor" is considered a better reason to have kids these days than, say, ancestor worship or nietzschean vitalism.
Because having kids then was a way to increase quality of life. The kids could be put to work from a young age and help make money. Now, with so much modern tech doing physical tasks efficiently, a kid isn't going to add much value and instead is going to be a money sink.