Why are there so many American phrases about derrières?
Many of your examples are synecdoches. "a figure of speech in which a part is meant to represent the whole . . ." New Oxford American Dictionary.So, "The army put 200 boots on the ground." "All hands on deck." "Twenty head of cattle . . .'
Why that particular part of the anatomy is chosen to represent the whole may have to do with its sheer bulk in the English-speaking countries.