Reason for handlebar diameter
You also need to consider how people use the handlebars. e.g. covering the brakes -- could someone with small hands reach the brakes easily with an optimal-for-the-average-person diameter (I suspect not, especially with the brakes of many years ago when standards were set).  Also if pure grip comfort were the issue, drop bars wouldn't have happened; instead they are optimised for a range of positions for different situations and to allow the rider to vary their body position for several reasons.  

It's easier to take a thin bar and make it thicker by double wrapping or padded gloves, but if your bars were too thick (small hands) they would have to be replaced, reducing the market for standard bikes.

Flat bar bikes tend to have grips with some thickness to them, taking them closer to the optimal while still allowing the rider to cover/use the brakes for long periods without contortions or spoiling the grip you'd need over rough terrain.

The exact numbers are a matter of convention, which may derive from imperial measurements, a round number minus a fixed thickness, or a combination of the 2 -- this is well documented for tyre sizing as a comparison.

This was a comment before I expanded and formatted it.