Most of them who stay put in Australia never really get to put their skills to good use. Our digital landscape is quite conservative & somewhat arguably boring, consisting of mainly traditional finance/banking & insurance - and we all know how these companies work internally (digital as a cost centre).
Don't get me wrong, there are a few startups floating around who are growing up e.g., Canva, Campaign Monitor, Atlassian (for which I'd argue is a US company), but even the bigger ones only hire a couple of thousand on-shore devs a piece. Most of the good devs with half an inkling go overseas when they finish Uni to earn good coin and work on far more shaping stuff.
I think we are risk-averse in Australia due to life being pretty good on average here (so what's the point?), which is fine, but more jarringly; the Government do very little to assist or incentivise entrepreneurs. I wish they did more, we'd see way more talent remain here and plenty of more ideas come to life!