It's not like Americans don't know the metric system, it's that there is no political will to change.
But it really doesn't matter. Large parts of industry in the USA are already "hard metric". Anybody in STEM certainly knows metric, because that's often what they exclusively use every day.
I suspect that that was the high point in teaching metric in schools.
There's a difference between knowing metric and really having a feel for it. I speak as someone trained in physics, but that doesn't mean I had a real sense for what it means. What clothes should I wear for 23C weather? Is 13mm of rain in an hour heavy or light? Is a car which consumes 4.5L/100km one that is fuel efficient compared to most? How many centimeters tall are you?
Even now, living in a metric country for years, I have to think about some of these by translating into, say, 52 mpg.
I used 'standard' for some time, and now live in a metric-only country. I have little practical knowledge of the weather forecast, for example. I know 9C is colder than 12C, but it is rather difficult to assess what clothes I need to wear for that temperature change. I have no real notion for how far 350km is compared to miles or travel time.
A 2kg bag of flour isn't so different from a 5lb bag of flour, and 2 liter cartons of milk look to be about half a gallon. But measuring recipies in deciliters? How big an apartment is in square meters? How big is a cm compared to my thumb? How can I estimate lengths shorter than a meter (which is approximately a yard)? How do I know if I have a fever? What is normal comfortable room temperature?
Some of these questions folks in STEM professions will have some awareness of, but it is definitely not something that makes us fluent enough. And I think the only way to do this is to switch completely.
I learned metric in school and used it in college science courses, but didn't become fluent until I lived in Germany for a few years. It was trippy having "non-science" people talking about grams and centimeters. Metric fluency has come far more easily than German.
Oddly enough though, I've found that I react like the Germans around me to weather forecasts in Celsius (30 degrees - it's going to be HOT!) when if I heard it in Fahrenheit, I'd react more like a Texan (86 - pleasantly mild for a summer day)
People were talking about this even in 2012 when playing Ingress: https://reddit.com/r/Ingress/comments/13vjcw/the_real_nianti...
Even as an American, it feels way more natural for me to express distances < 1km in meters, not "yards" (seriously the most wtf unit) or feet.
Now I really have no preference on feet-meter scale other than the fact that I find the need to fall back to decimal meters or tens of centimeters when I can use feet for a more round number. I'm terrible at gauging distances, so I have very little use for either without a direct measurement.
