So... what you're saying is - under your system - we could all just not bother with our classwork for the entire year and just put all our effort into the final HSC exam. And how would people who were sick on that day go? Well, obviously they can just refer back to the school marks to see how well they might've gone - but oh wait, there's no weighting on that and there's no reason to do internal schoolwork because it counts for zero anyway! So who would've done that? No one. So maybe the person who would've gotten 100 in MX2 but didn't because she was sick on that day fails and has to repeat the HSC.
Now that's not fair.
This system in place now is fairer as it rewards constant work through the year. Do well internally and get a good rank, do well externally and have a good HSC mark to combine with the internal moderated mark.