Because a negative test doesn't mean you don't have covid and won't spread it? - It just means that at the time you were tested you weren't shedding enough virus to test positive.

For example - I live in the US now (originally from Melbourne) - my husband was infected with covid on Thanksgiving (we still don't know the chain of how the person who infected him got it) - a week later he was sent home from work with a high fever (40.5C) - I took him right to get a test and it was negative. 2 days later he got another test that was negative, and 2 more days another test was negative.

The day he got his final negative test was the day I started feeling sick - tired, rundown, vaguely warm. I got a test immediatley and it was negative. Day 3, 5, and 7 of me feeling ill were all negative.

17 days post exposure for him (and 8 days after I first started feeling ill) I lost my sense of smell and taste - and that was the day I got my final test and the only one that ever came back positive.

2 months later my hubby donated blood and he was positive for antibodies.

Same reason as the "must have a negative PCR test before flying" thing doesn't really work beyond its design to make lazy people think "oh this is too hard I can't be bothered" - you can go get a test then spend the next 3 days getting wasted in 30 different clubs and get covid that way and then having the test taken previously means nothing.

Don't get me wrong, I think the current restrictions in Australia are a little OTT but IMO they're way better than the way the US and UK just let things rip.