The British don't sound like the British either.

The "British" constitute four different accents (Irish, Scottish, Welsh and English) with very many different sub accents and sub dialects. You can find different accents within twenty miles of different towns; different regions and counties of England have accents that sound VERY different. An Australian with a received pronunciation accent sounds more similar to an Englishman with a received pronunciation accent than either do to someone with a very strong Teeside or Liverpudlian accent.

You are asking - "Why do some English people who went overseas speak differently to the English people who didn't?" The answer is the English people who stayed at home don't speak the same either: even in England things like rhoticity/non-rhoticity, flat vowels, et cetera are split half way down the population.