Why can't Southeast Asians and Polynesians understand each other's languages?

Many Southeast Asian languages, together with Malagasy spoken on Madagascar and the languages of Micronesia, Melanesia and Polynesia belong to a huge linguistic family that spans three quarters of the globe - the Austronesian language family.
Despite deriving from a single ancestor spoken thousands of years ago, can speakers of languages separated by millennia and thousands of kilometres of ocean still recognise similar words and phrases in their respective languages?
The truth might surprise you.

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