We’re not dumb

I’m not suggesting this at all. I’m arguing that using “parts per 10 million” makes more sense to an American audience because fractions are more common in America. So using non common denominators is easier for an American audience. Whereas in metric countries, using standard denominators like thousand, million, billion, etc with decimals and significant figures is easier to interpret.

it’s a simple fact that comparing 5 to 20 is easier than comparing 0.05 to 0.2.

Sure, but you’re ignoring the additional cognitive load of using non common denominators. And losing the ability to easily compare these metrics with others.

Every engineer and scientist in the US uses metric

Imperial measurements are commonly used in construction & civil engineering in the US.