OK, so you just can't turn on the auto-land system, point it to the appropriate runway and let it do its thing on a flight from LAX or DXB, then?
Aviation is all about options. We do everything possible to keep as many options open as possible. Fuel is the greatest giver of options, because it allows the luxury of time (or miles). Departures from LA to Melbourne do not have an alternate to destination as a matter of course. If the weather is good, that's a waste of about 10 tonnes of capacity, that is rarely needed. This only becomes relevant towards the end of the flight, as for the most part of the journey, you have the choice of many airports within (potentially) thousands of miles. In fact, for most of the journey, you'd be dumping many tonnes of fuel if you had to go anywhere else.
Departures from LA (to Melbourne) are almost always at maximum brakes release weight. The chances are that we simply cannot carry any holding or alternate requirements for Melbourne, without offloading a large number of passengers. And most likely we won't actually need the fuel at the end, as the prediction that we have to use is about 20 hours into the future..they get better as you get closer. So, in simple terms (shock, horror), we may depart LA without sufficient fuel for the mission.
But....
Along the way there are a number of primary alternates, with the last one being Sydney. As long as we keep our fuel reserves above the line for any point that we could use (at all times, with no gaps), we can continue. If you can keep it above the line to destination, then you get to go there.
These diversions which occur at MEL must cost the airlines a fortune.
I'm sure they do. But that's actually cheaper than carrying "fill 'er up" fuel everywhere. How often do we miss out? In 32 years of airline flying, and untold numbers of LAX-MEL, I've diverted once. Come close a few times, but doing those numbers (over and over) is part of the job.