Every child in the US is required to learn the metric system before graduating from High School because it is required for science classes. They start learning it in elementary school. Realistically the metric system makes a whole lot more sense.
The question WHY should be WHY are Americans so determined to be stuck in a system of measurement that is not used in most other countries? The scientific community agreed to only use the metric system long ago. American money is divisible by 10 but inches are by 8 and feet are by 12? No wonder our children fall behind in math after 4th grade. They are required to memorize doofy measurements. Ever wonder why we were required to memorize multiplication tables to 12?
I'm fortunate. Besides living overseas, my degrees are in chemistry and physics. I automatically convert in my head. We all do it without thinking. It just depends on what you are comfortable with using. There are 39.25 inches in a meter, 2.54 cm in an inch, etc. I find it easier to work totally in metric. 100 cm = 1m, 10 mm= 1 cm, ...
When I'm squaring awkward measurements I convert to metric. It is far better to be off by 1 mm than 1/8 inch.