This is too precious to not share :3
As for a plausible answer: Humans tend to stay in a place once they’ve set up housekeeping. Hell, there’s this town in the US with an underground coal fire going on, and there’s still a few peeps hanging around there because they won’t move.
Now for the details:
First, we have a group of people who moved in because the hunting was incredibly good. They immediately rendered extinct all the megafauna and repeatedly burned all the forests, thus making Australia the harsh bucket of arseholes it is today. [Papua New Guinea helped a little by rendering the continental climate extremely dry FYI] But they stuck around. I assume because (a) it was too much bother to go back and (b) by the time the can of arseholes happened, nobody alive remembered where ‘back’ was.
Next, we have a group of people who were essentially dumped there as human rubbish - and the guards whose job it was to ensure that prisoner life was at least three cans of arseholes. They pretty much instantly fucked up the local ecology by introducing all sorts of critters by accident or design because turning Australia into a cheap copy of Europe was seen as desirable.
Loads of them stayed because they couldn’t afford to go back.
THEN, among the free settlers [Because there was loads of room for sheep once the rats got the ground-nesting birds] we have a group of people who came over because of the many untapped resources just floating around. Translation: they were after the gold, those greedy bludgers. They instantly found that looking for gold was an extra can of arseholes on top of, you know, just living here.
Some gave up and farmed either cattle or sheep. Some fucked off when the gold rush petered out. Some stayed because they were still poor as fuck.
And by then, there was trade and infrastructure, and all that kind of nonse so the existing population pretty much said, “What the hell” and kept living here.
Mayhem says: “Because we’re tough as nuts, we eat nuts and bolts for breakfast, and we’re made of brass tacks.”
Use whichever answer suits you.