Good Observation, but reality check time.
First, Brisbane and Sydney suffer from something much of Mel, Adel and Perth don't, they are called hills. Look at Adelaide's Belair line compared to the other lines in sectional times, average speed and average speed as crow flys. One of Mel's lines conviently stops at the foot hills and in the days of old the line continued as 2.5' gauge and indeed still does into the hills. In the SE, they just closed it.
If you go to the new lines in Brisbane, something Syd lacks, you will find indeed they are 100-160km/hr from station to station. ie GC line past Beenleigh, new section of NCL from Cab to Beerburm and I assume and yet to use the new Springfield line. The GC line has a few sections the trains pull out from one station and go straight to 140 and don't pull back until next station 4km away, even Perth cannot match this.
For the Brisbane Airport line a few issues. First the Viaduct, yes its new, but had to be built with existing constraints so apart from initial bend away from main it has a few more bends to avoid roads etc. Top end speed is I think 90 (but may have been 100). Its also on a Viaduct, so calls for a higher level of safety and maybe same alignment on ground would be faster (I'd rather take 1min longer than be the first to ride a EMU into the dirt). From main heading to City you hit the tight bend at EJ, which is also a junction, so flat elevation of outer rails prevents higher speeds. Then onto BH, again bends built with pick and shovel 100 years ago to go around hills and often a suburban in front doesn't help either. Once you get past the slow BH, things get better to Central. South Side Beenleigh line is full of bends.
Much of Perth's rail infratsructure is basically green field or rebuilt run down brownfield. Wind the clock back 22 years and you had old DMU's, a closed Freo line and some of the other lines didn't even exist. With the rapid growth came some smart planning taking advantage of the corridore down the middle of the freeways, which are built on flat terrain. Something Brisbane could have copied to some degree but didn't.
Also older lines built when milk was delivered to local station by rail and by horse and cart from there have stations less than 1km apart. The newer lines are building the mega hub stations with large parking, bus stations, kiss and ride etc etc, but these stations are on average about 3-4km apart.
Do the sums, with a acceleration and braking effort of 1ms-2, and stations 1km apart, you don't get much saving going to open end max speed. For the record its 113km/hr for 500m.
Yes it would be nice to see some of these lines straightened some what, the gain is fairly minor for the cost and at the moment there are other priorities. Such as capacity constraints in city, Spingfield line extension, GC extension, Further dup and realignment on NCL, SSC line construction, KR line construction. plus more trains as 79 x EMU's will need replacement from 2019, upgrade of remaining timber sleepers and light rail and hopefully new rebuilt line to LV.