There is a confluence of many factors. I work in the tech industry, so my experience is from there, but also more broadly;
- There are far less O/S students coming from China (by far the top consumer of AU education), because the government there has been literally saying AU is an unfriendly country and to not go there. Traveling internationally for Chinese people is also much harder now, and there's not any guarantee government there will approve outbound visas. This is a political set of decisions, and you can look at it as the Chinese government not wanting their consumers spend money in countries such as AU.
- There is a massive lack of housing. Even if workers were coming here in droves (and let me assure you that they are not), the rental vacancy rate is the lowest in history. So where do you expect them to live?
- Many workers (skilled) left during COVID and are not interested in coming back due to various reasons
- There is a natural brain drain where a lot of people leave to go to EU or US, because we have a shrinking set of skilled industries (for example, aeronautical engineers is a salient example), and nonexistent in many others (eg. AI).
- This is a multiplier for both of the above but, for young skilled workers (the kind we really want), they see AU as a country with very high cost of living here, low quality of life (subjective I know), very boring in terms of events and nightlife, very complicated and expensive to travel inside the country and stay in accommodation (compare with cheap bullet trains all over Europe), and there's just no real reason for them to come here.
Historically, AU has always had "high" salaries, which helped attract talent, however lately, at least in tech, remote work and very big tech companies have balanced the equation a lot, and you can now get very good salaries in many EU cities, which are arguably much more livable than AU - notwithstanding Europe's incapability to build reliable energy production or air-conditioning. Combined with the fact that everything here costs about 3 times more than in any other developed country, the money equation no longer adds up in Australia's favour. I know far more people who are looking to leave AU, than friends overseas who are interested in moving to AU (literally none).
In Europe, you can go and have a night out as often as you like. In Australia, unless you are earning top 1% of salaries, it's a monthly activity which will be very expensive. Add on top of this that Australia is a totally overregulated nanny state which makes the EU look like a libertarian paradise.
Edit: Forgot another factor - COVID expedited the retirement of the baby boomer cohort heading towards retirement. This is a very large group of workers that won't be possible to replace - we need to work our way through that with automation and efficiency gains.
