You would be surprised. But that is why we supposedly live in a democracy (after all, it wouldn't be so if the rich and big corporations could just buy votes, if they already do so anyway).
Direct compensation of people affected in order to move only addresses a financial concern, not social or moral ones. Moreover, appropriate compensation (even at veritable market rates) is rarely afforded to such affected people in such situations.
One can also make the argument that the airport has expanded multiple times and people had different expectations of the noise during different phases of its expansion. For example, someone who has lived in the area for decades was probably used to only a few planes compared to now, albeit they were louder (but probably didn't land at night). If the same persons were living in the area, the traffic has increased and hence they did not move into the area to put up with the new level of noise. It's actually almost equally facetious on both sides if you analyse it carefully (i.e. the other side of the coin is that it is impossible for one to reasonably think that there was no possibility of the airport expanding its operations). For those that have just moved into the area, perhaps the caveat is more valid, but people are people and they still won't take it.
Moreover, it is both the airport itself and flight paths which are the bones of contention for noise in SYD. If it were possible to always land on the third runway, for example, then apart from necessitating some probably wasteful but extensive flight paths to avoid residential areas, then I don't think a curfew would be necessary. Unless wake noise and so on from the airport carries a fair distance, which may be another problem. Also, even if it were possible to instigate this scenario and remove the curfew, it'll only take one go around and there will be a plethora of complaints to the local MPs office tomorrow.
