Modern parenting is hard because we lack data points.
Our ancestors parented in villages where there were lots of kids – lots of walking, talking data points.
Without data points, it is impossible to gauge what is “normal.”
With one kid, we rely on Google searches or stalk our friends’ Facebook profiles to see when their kids started crawling/babbling/pottying.
With two kids, “normal” development exists linearly, on a spectrum of more/less, slower/faster, sooner/later.
Introduce a third child and you start getting unforeseeable “outlier” behavior.
Introduce a fourth (and more) and you realize how large and wide a spectrum “normal” actually is.