It isn’t. Topic title should be Why is American English different from English English? There is no ‘British’, as it would imply the Scottish and Welsh had a hand in it, while the latter borrowed it later.
Beings as America was one largely borne of convict musings, they tended to speak improperly and cut short real words… saying what they saw if you like and pronouncing incorrectly… ‘leisure’ for example. Much like how some dialects roll, such as how the clipped southern Thai dialect differs from Central.
Oftentimes American English is easier and more logical in that regard, but please have some respect for the source. 😋