Supply and demand. For people that want to stay in Sydney – UNSW, USYD and UWS are their only options (JMP is in Newcastle), and because there are so many applicants each year, and the academic culture being quite competitive with so many selective schools, cutoffs are high not just for UNSW but for all of them.
Yeah, ATAR requirement for USyd = 99.95, and, historically, UMAT requirement for WSU was around 95th%ile (non-rural, non-GWS).