It’s a constrained space thing, not an “Australian thing”.
R__ is right to say the oval shaped Gabba is very tightly wedged in a densely populated residential area with a heritage school building on one side.
There is the space only to expand capacity marginally.
But what it does have is:
has a mass people movement public transport hub with an underground train station (circled in green below under construction),
Brisbane Metro connections (very long new electric articulated buses that look like trams, but have rubber wheels and drive on roads and dedicated busways).
suburban bus interchange,
a very central scenic location not far from the Main Press Centre, International Broadcasting Centre and competition venues at South Bank and the Brisbane CBD.
The Gabba, Brisbane
The Queensland and Australian Governments are working out the funding split as we speak but I cannot see that the 2032 Ceremonies and Track and Field will be held anywhere else except the rebuilt Gabba.
Yes it is expensive. 50,000 is one of the smaller capacity Olympic Stadiums but as a legacy venue after the Games, 50,000 will suffice for the legacy sports tenants, cricket (summer) and AFL football (winter).
Allianz Stadium, Sydney
The new Allianz Stadium in Sydney is also in a tight space wedged between Fox Studios, Venues NSW, Sydney Cricket Ground, University of Technology, Moore Park Road and Driver Avenue.
45,000 is about right for the legacy needs of this rectangular venue whose sports tenants are A-Leage soccer and Rugby League. Is served by two light rsil lines and a very busy bus corridor