It is not an entirely correct statement to say that the US has not adopted the metric system.

Also many other customary units are in common use in other countries.

The US authorized use of the metric system in 1866.

the US joined the international metric convention in 1875.

In 1893 all customary units were officially defined by international metric standards.

It has been used for science and science education at least from the 60's.

In 1975 it was adopted for use by federal agencies and the military. A metrification board was set up. I saw some federal road signs in Kilometers in the late 70's. (I bought a new dodge van in 1977 that had a metric speedometer) This was stopped by Reagan in 1982.

In 1985 metric was made the preferred system for commodity labels.

Since 1994 commodity labels are required to include metric measure. Currently, metric only labels are legal, avoirdupois only are not.

You will not find a single SAE fastener on a new car built in the US. The automotive and electronic industries are entirely metric, as is anything made for export.

Some states have road signs in both systems.

Though Britain may be 'officially' metric, they do not seem to be changing their road signs over in much of a hurry, and still measure their beer in pints, and themselves in stone. Canadian lumber does not seem to have changed from customary measure. Canadian food etc. labels seem to be still using dual denomination. (bilingualism is in the constitution, right?)