Why are people in Japan Asian but not people in Australia when there are populations that share DNA?

Hello Dear Smart People,

I’m Australian and lately have been learning more about our history and the First Nations peoples of Australia.

One of my favourite facts about Australia to share is Torres Strait Islanders, an indigenous peoples to Australia are melanesian peoples who’s roots stretch from Japan to Papúa, to Australia which is why we share similar features and names sometimes.

But than it got me thinking, why are Japanese people counted as Asian while we share DNA, family and culture with those same people, while being in the Asia pacific region but are not counted as Asian?

I understand theres no set worldwide agreement on continents and geography, but if anyone could share some light into what info im missing I’d be more than greatful.