Were American, Australian, and New Zealand English dialects ever spoken in Britain before the colonization of these lands?
Languages change. Otherwise, we’d still be speaking like Chaucer. The British settlement of America started in the 17th century; there has been lots of time since then for several different American dialects to develop. The British settlement of Australia and New Zealand started over 100 years later, which is why these dialects are closer to those spoken in Britain.
There are aspects of American and Australian dialects which were brought over from England, but which have since been dropped from British English. And there are aspects of American and Australian dialects which developed in the colonies.