- I am just guessing at this, but I have not heard of any evidence that any culture had a large percentage of parents that wanted their babies to die. Thus the “good parent” ratio is probably not changing a lot across societies, at least when compared to the differentials in infant mortality that exist across countries and time.
- “Access to health care” is part of living standards.
Usual causes of high infant mortality are due to diseases that result from inadequate nutrition, lack of clean water, or diseases that might be caused by inadequate sanitation. These are all obviously related to living standards.
I believe the reason why it is useful is that it is a quantitative metric that is easily tracked, and has been tracked for a long time (birth/death records).
One can imagine more powerful descriptions of “living standards”: but is there any way to measure them, particularly in historical data? That’s why we have to find indicators that can be reliably calculated, and which are expected to correlate with fuller descriptions.