Why don’t Asians in Australia have the Australian accent?

As other respondents have said, (a) it depends, and (b) they do. Reflecting on the Asian Australians I’ve known in the past thirty years:

• People who’ve come off the boat naturally aren’t going to have an Aussie accent. Duh. Although I’ve spoken of a counterexample here: Nick Nicholas’ answer to Who are some people you know who became fluent in a foreign language as an adult?
• Second generation Asian Australians will, by default, have an Aussie accent. Of course. I haven’t noticed someone who doesn’t.
• In fact, I contrast second generation Asian Australians with second generation Greek Australians. There is a distinctive Greek-Australian accent that I can pick out in 40 year old and 50 year old 2nd gen Greeks: it’s not Greek at all, it’s overcentralised, and a little overenunciated. I don’t recall something similar with Asian Australians. But as I keep protesting, I do have a tin ear.
• Some Asian Australian schoolfriends and acquaintances have been on the Cultivated Australian/Vaguely British side. That correlates with Taiwan (not sure how), and with Hong Kong (a bit more obvious how).
• On the other hand, my fellow engineering student from Shepparton in rural Victoria, predictably, had one of the more ocker accents I’ve ever heard. Mate.