QF does have some lower fares, for example the Circle Pacific fare would be less than that, as would any of the RTW fares. AA also codeshares on LAX-SYD, which might reduce the fare if you have a corporate discount with AA.

As to why QF costs 50% more than UA, the answer is supply and demand; lots more people are willing to buy the QF ticket instead of the UA ticket. Whether it is worth it, or good value, is a different question. Personally I've chosen to fly QF 99% of the time on this route, instead of UA.