How did the American and Australian accents evolve?

I’m not the first to be intrigued about the origins and development of the American and Australian accents, despite their strong British and European influence. The history is further complicated if you believe those 2 continents were discovered before the official ‘discoveries’ in the 15th century onwards. Did the Ruling Elite deliberately create those accents to differentiate them from the ‘enemy’ (Britain and Europe), to foster a new identity?

If the Australian and American accents evolved from Britain, predominantly, then where’s the similar-sounding Australian and American accents in Britain? Why would a 18th century British person travelling to Australia (for the first time) change their accent to a new one if there’s no native ‘Australian’ accent to change their old accent?