Because the government has (rightly in principle, but poorly in execution) determined that a slightly open border (for controlled return of expats and continuation of flight crews for freighters etc) outweighs medical risks of a virus escape turning into an overloaded health system. The mitigator is lockdowns.
Re funding - How many ICU rooms can you build and equip from scratch in - say - 6 months? Or maybe 12 months? And how many frontline health workers would we have spare if it spread like Europe or USA? Would they have been built in time to stop the health system being overloaded? With a huge country like AU with many regional areas remote from hospital facilities it would be a disaster. That's why many communities have been off limits. In WA you fly a pregnant woman the same distance as London to Budapest to get to hospital if she goes into labor more than 4 weeks early.