Because no system is perfect. A few cases will be missed and...

Because no system is perfect. A few cases will be missed and that's all it takes to start the spread. We were protected before by the closed borders.
Queenslanders got an easy run because of the protection we got, especially up here in FNQ where Covid has pretty much been a non-issue other than the economic impact on international tourist operators.
Next year we will start living with Covid for the first time. I think all restrictions on anyone will gradually be lifted, probably after we hit 90% fully vaxed.