There are many (not all) people from random government schools (I know I'm generalizing here) just don't really care about their academics. I went to an average government school before going to MHS, and it's not that the students were "dumb" or anything like that, but they just didn't care about study. Some of my friends are now doing VCAL, and the most of the rest aren't really that worried about VCE. Many of the top students got into MHS/Macrob, and the people remaining wouldn't really be affected all by this, but the people who got into MHS/Macrob gain a significant advantage in their studies.
If students don't care, then they won't really be affected by anything. People don't always need high VCE marks, there are many many other options. Why do students who want to pursue academics have to be hindered by those who want to pursue alternative (and just as viable) careers?
There are many high performing students who cannot make it into the 2 selective schools (3% rule!),so may as well make more selective schools to let those who wish to pursue high marks get the best environment they can get.