Because the Department deals with hundreds of thousands of applications every year. Most of the processing time is literally taken up by the application sitting in a large pile waiting for a Case Officer to be assigned. And if there is investigation to be done, or further information to be sought, then it will take longer. Once the application has been assigned, it is quite possible for a visa to be processed in a single day if it is well-prepared and all of the requirements are clearly met. But it still sits in the large pile first before they pick it up. Please read this: https://www.downundervisa.com/2013/12/04/australian-visa-application-taking-so-long/