@BeyondOverTheMoon not being the one who originally thought you couldn't fly directly between 2 Southern Hemisphere points, I'm only offering my thoughts .....
Part of the issue of CT in general is that none of us are experts, or even mildly knowledgable about everything. And the majority of time it doesn't matter - I'm no civil or structural engineer so have no idea how buildings stay up or re 9/11 how they "should" burn or collapse or maintain integrity.
99.9% of the time it still doesn't matter .... but take a 9/11 event and somehow we're amateur experts. When it doesn't add up to us - who knew nothing in the first place, we're intrigued. Then when you get 2 "experts" who seem to contradict themselves, who do I trust if the supposed experts don't agree. Then the CT develops.
But, every day there'll be a building collapse or fire or explosion that never makes it to the mainstream news, so I never hear about it, so I never have cause to query it, so no CT.
So for the flights ..... if you'd heard that no flights exist on those routes, it's reasonable to believe at first because you've no reason not to believe someone you trust who told you that. Only if it sounds untrue and / or you've reason to research it (because you want to fly from Cape Town to Perth or Rio) would you learn it's not true, or not totally true, or there's some truth.
Making an assumption here, but perhaps you hear that there's no direct flights between two specific places in Southern Hemisphere which may indeed be true - and you file that away as "no direct flights anywhere in SH". Only when someone challenges you, do you find out different.