I used to think the same thing but read that it was the deaths of kids that brought the “average” lifespan down. Check the numbers again.
Of course the death of children in 1900 brought down the lifespan of children then! That’s exactly the point I was making.
Also, the death of women in childbirth brought down lifespan. The death of people due to infections brought down lifespan. The death of people due to accidents brought down lifespan. My own uncle died at the age of 8 due to appendicitis. Three of my great-grandparents buried children that never would have died of those illnesses today.
That is what took people out in those days, they didn’t live long enough to get cancer. Something else got you first at an early age. These days we have excellent prenatal care (at the age of 56, I don’t know even one woman who has died in childbirth), we have antibiotics, outstanding hospitals and trauma centers, etc. What people died from back then would be handled much more differently these days and would not result in death. These things took them out early before they could ever develop cancer.
Take these reasons for early death in 1900 out of the picture, and what, then, remains to cause death? Heart disease, strokes, cancer, catastrophic injury ....
You just cannot compare cancer deaths in 1900 to today. It’s a totally different world with regards to our medical capabilities.