Because that's where we keep The Outback!

most of it is desert. only the right edge and a little of the right bottom are green and the bottom left tip are green, and some of that bit on the top left

the land has been above the water for longer than most of the rest of the world... the only volcanic parts are what i just described, or if they aren't volcanic, they have a lot of humidity blowing in from the sea

i think that there is good reason to suspect that not just australia but also antarctica were face forward at the time a massive X/gamma/cosmic/etc blast from a nearby supernova went off, and the whole place was sterilised and just turned into dust and eventually blew away

idk where the idea came from exactly but something like 40k years ago this likely happened

the main point is that there is very little carbon on the land, at all...

and maybe after it got sterilised maybe it moved way south so it was out of the way of the big flows of water which might have helped plants colonise the land, but that never happened in the last 10k years or so