Because it's the best of the best. Australian's don't find international games entertaining at all, they are boring.
Yes, because us Australians are all the same person.
Perception is reality and the fast majority of Australian fans (where by far the best competition resides and the money resides) see SOO as more important or more interesting that test footy.
Having said that I would disagree that SOO is the pinnacle, it is the NRL or more accurately, winning the NRL Grand Final.
I don't know anyone that wouldn't trade a SOO victory or World Cup victory for their team winning the premiership. You can see that the players feel the same too.
It is what it is and to be fair very few sports have internationals as their true pinnacle. Cricket is one sport that does have it because internationals are a full time job and that is what people watch.
Well that's why this argument exists, people define 'pinnacle' differently. The highest level of footy is Internationals, by definition. As for highest quality, that's debatable. Most Australians would probably say Origin. Some would say the NRL finals, as these are professional teams that have trained and played together all year, rather than just a few weeks like in rep games. I would say Internationals, not only because all the best players are available but because there are a variety of different playing styles you do not get in NRL or Origin. NZ v Tonga last year for example - the entire experience is something you can't get in Origin. You also see the best of the best - the Australian team of 2013 is the best team I've ever seen at any level.
Or you can define it as what players and fans desire most, which for most would be winning the Grand Final, like you said. To be fair then though you also have to include Super League in that, as I think most English fans would rather see their team win the ESL GF rather than see Roosters win the NRL GF. You can also take that further though - to me nothing would match winning the competition with my local club side. I'd much prefer to win a GF I played in for my hometown than see Parramatta win the GF or NSW win Origin. Does that mean the Proten Community Cup is the pinnacle of rugby league?