seems like the obvious answer is, "why do we americans call it a 'trunk'?" For that matter, why do we call a hat a hat? Why don't we call it a bill?
The stage coaches in the American West had a luggage storage area in back of it. It had a leather cover over it, and was called a boot.
Why do you call the boot of a car a trunk? Regional differences in language.
because its at the back
I am not British but is the correct term not what they say it is? I mean you are using the English language you should spell it the way they do should you not also?