Advantage is certainly subjective, not to mention it's not always immediately observable. (I'm not actually trying to defend the GSP, or suggest there is always definite advantage for buyers with the GSP, as I know it has some problems, just pointing out that things that advanatge sellers aren't necessarily a disadvantage for buyers. I get that a lot of people avoid it, but also understand that some sellers are well aware of that).
To put that in a more relatable context, as a sellers there are a lot of things I do that make my job easier and simpler. The more I can do to make my job easier, the more I can do for my customers in other areas (eg I would really love it if I could just block people from providing me with a parcel locker / collect address. PO boxes are fine, because all mail sent via Aus Post can be delivered to them, but only parcels can be delivered to the other types, and most of the packages I send are not parcels. This creates a huge headache for me and for buyers who don't know, for whatever reason, they won't get their package delivered to a locker. Either I have to get a deliverable address, or send it off anyway and hope it gets redirected ok (because they almost always get redirected to an alternative address, or just sit at a PO awaiting collection). If I could just prevent these addresses from being provided in the first place, I'm sure buyers who use these addresses would see it as an inconvenience, but not anywhere near as much as the resulting mess if they provide it. I'd much rather invest the time it takes to sort all those out each week was invested elsewhere.
Similar principles can apply to the GSP, while some sellers aren't aware and/or don't care about the higher costs, and other sellers wouldn't ship overseas at all if it wasn't for the GSP so the items wouldn't otherwise be available. There are a couple of countries I won't ship to without full tracking etc (up to 10 times the cost of economy air mail) but since I have quite a few unique and exclusive items, it's better for all if I at least make them available rather than refuse to ship there at all.
The other thing is, for those who live in countries who do pay import duties and charges, it's all taken care of at the point of purchase (no more mail being held to ransom, no more surprises) so I suspect that just because Aussies don't (yet) experience that on packages under $1k, doesn't mean it's not a great advantage for those in other countries. (Not to mention it eliminates stuff like buyers messaging to say they want refunds because they didn't know they had to pay import charges, or even refusing to pay them and claiming non-receipt, both of which have happened to me as a seller). Sometimes... A seller just has to do what's right for them.
