I'm pretty sure that while many ruse students do medicine, the majority don't.
The primary reason most choose UNSW over USYD medicine is because UNSW offers a guaranteed direct undergrad degree. Unless you get 99.95, getting into USYD postgrad med isn't guaranteed since you need to sit the GAMSAT's. Also, USYD med is one year longer.
As to why many ruse students choose medicine (and also law), the reason is pretty simple. They choose those courses because it's a high cut-off, so it's prestigious. Maybe their parents forced them to, maybe they felt pressured, maybe they want the bragging rights and feel respected, or maybe they felt like it's a waste of their ATAR if they do anything else. Either way, the vast majority of them do it because it's prestigious. The proportion that have a genuine non-ATAR-related interest in them is a tiny fraction of the cohort - you wouldn't expect it to be vastly different to the proportion of students that are interested in say, teaching, or science. If you dropped the cut-off of med/law to 70, I guarantee you most of them would be jumping ship to the next highest course. Many students that do med (or law) do it for this simple reason, even though they themselves might believe that they're different and not part of that "prestigious" crowd.
Once met someone who said they really wanted to do a combined med/law degree. What a coincidence right? Wanting to combine 2 degrees that just so happened to have the hardest entrance requirements. Like c'mon, don't kid yourself lad, you don't actually want to do it, no matter how much you try to convince yourself otherwise.
FYI all the top ruse kids in my year who were on the IMO team went on to do math (or a math-related degree) at uni because they were genuinely passionate about it, so not all the bright ones go on to do med.