I think you're misunderstanding how client/server games work, not to mention massively underestimating just how much client trust FFXIV has put in the player over the years.
Back in ARR it was possible to tell the server you levelled up through handins and such. And no, I'm not even kidding, go dig around and you'll find videos of RMT bots quite literally levelling themselves up multiple levels nigh instantly whilst merrily stood around in town.
Suggesting that everything has to be approved via the server is also wildly missing the mark, have you missed all the RMT teleporting around usually under the world? Go stand by the Uldah Aetherite and you'll generally get a bit of a rude awakening, if you don't see any activity there, again go look on YouTube. Countless examples.
IMHO the issue at the core of FFXIV's netcode is as I've said many times, it boils down to cost savings on SE's part. A slow tick and update rate to keep server and connectivity costs as low as possible. SE have history here and they've always done their best in MMOs to reduce server costs wherever they can get away with it.