This is because banks have a sizeable proportion of their liabilities – such as retail deposits – that do not reprice in line with BBSW rates. Also, the bulk of non-banks’ loans are funded via RMBS issuance, and the cost of issuance has risen by a bit more than BBSW rates (Graph 12).
The motivation for granting such independence is to insulate the central bank from political interference, especially interference motivated by the pressure of elections to deliver short-term gains irrespective of longerterm costs.