Think your missing the fact the due to a cancellation the aircraft and crew could be in the wrong place, so it could still have to be flown somewhere empty to place where it needs to be to maintain the schedule. So crew time, airport fees, any overnight allowances and fuel are potentially still payable.
Pretty sure you are not in the airline industry asking these questions. Domestic crew and aircraft schedules are multiple sectors, a crew and plane wont be stranded from a single or even multiple cancellations, planes, crew, stranded in destinations only happens from breakdowns or other unforeseen occurrence. As far as the financial arrangement you are talking of, that's purely if you book through a travel agent, which during covid was very hit miss if you'd ever see the money again, even if your airline did offer refunds. As far as I know all Australian airlines now offer refund or defer tickets if the flight was affected due to covid, also I think Rex was the first to make a deal out of that, forcing the others to follow.
I had some travel options booked directly with operators that were cancelled and they gave full refunds. I heard of travelers booked with the same operators who were booked through travel agents who struggled to get even part refunds, only offered deferrals etc. Travel agencies often get commissions directly from the operator, so refunds become murky and funds transfer is obviously handled differently.