That's because when most Aus airports were designed, international transits were not seen as a very likely occurence, due to our geographic location (and the SYD centric dogma of Aus aviation). Even now, they make up only a tiny fraction of pax numbers, and so don't have a great deal of resources thrown at them. As for rescreening, whatever you think of the effectiveness or not of our screening process, if pax who have/have not been through the screening process of another country are to be allowed to mix with those screened to our requirements, then they too must be screened to our requirements. Whether other countries are happy to accept the intermingling is up to them. Every country applies screening rules differently, so I guess we are just trying to keep ours consistant.