They don't do it because the U.S. has good standards for what is a crime.

For example cops can't tailgate you then give you a ticket for speeding.

Additionally, doing this only on freeways would probably make the roads less safe as drivers would go faster on surface streets. Thr 65mph speed limit was set in the 1950s by 1950s white men for 1950s cars. Freeways are safer than surface streets by multiple factors of 10.