The same reason why the British still use miles and stone. For some other archaic units still commonly used see horsepower, nautical mile, BTUs, acres, shots (volume), and knots (speed).

Most people use the units they grew up with or use every day as their primary colloquial units. If you grew up using inch, foot, or yard, and enough people around you can also use the unit, it doesn’t change anything in your day to day to continue to use them. It also doesn’t make sense to change what you use and already know if that is also what the people around you use and already know.

That said, Americans do know metric units and many use them every day, they just don’t typically use them when talking to other Americans. If the basis of your argument is US produced media then it just goes to show you don’t really know anything about everyday US culture. Also, why would US media, made for a US audience, with US characters use a unit that most Americans don’t colloquially use?

Complaining about US media containing Imperial Units is like if I watch a Spanish movie and complain about people speaking Spanish instead of English.