For SYD, you still have to clear immi incoming and customs, to get to the dom terminal, and vv from the dom terminal to the int one, which adds time.
So it would be complicated to do immi and sec secreening at the dom terminal, so your ride from SYDd to SYDi can be on the secure side, to save time.
As of now, even if the service between the 2 was free, its the time taken/time consumed that is the problem.
SIN Changi on the past immi side, for the driverless train, its easy, as security is at every gate, where as SYD int and dom is a one point only, and every pax flight into and out of Changi is classed as int.
KUL is half way, ie, the int side you have to get the driverless train from the dom terminal.
For SYD, there would need to be a specific authority to issue "free" terminal transfers, as its not in the airport's line of responsibility to provide "free" transfers.
If ever a "free" transfer train service could/would occur, it would need to a mid jointed TransportNSW sort of authority, ie, show your BP and get a "free" one use one way transfer opal touch card.
As of now, what with TransportNSW making money for every int - dom train transfer, and vv, they would not ever change.
$4.99 train transfer, $2.56 is fare, $2.43 is the gate fee.
Bus transfer, is just $2.24.