So the VCAA creates the exams, monitors the exams, marks the exams, analyses SAC marks and gives out study scores, but then the VTAC manages the ATAR? Why can't the VCAA just do the ATAR if they're going to do the study scores anyway?
What you need to understand that VCAA as a body is in charge of administering the VCE, which obviously includes study scores. At the end of the day, you complete at least 4 subjects satisfactorily including an English, and you receive your certificate, ie. you graduate high school. VTAC is the governing body for university admissions, and the ATAR is a tool used for admissions (much like the American SAT is used to pick students there). The thing about an ATAR is that it's not VCE-specific, VCE scores are just one of the many high school systems that can be used to calculate an ATAR, the others being things like the HSC and the IB for example. Anyway, if you get what I'm trying to say...it's like asking why don't the police just decide the sentences criminals get - the judicial system and the law enforcement system are separate entities. VCAA and VTAC are separate in a similar way.
EDIT: That's why VTAC does scaling, too. Scaling has nothing to do with how you went in your subjects/passing VCE, and everything to do with the calculation of ATARs. Scaling exists for the sole purpose of calculating 'fair' (some question this) ATARs.