We all get the theory. In theory this is equivalent of having a shared MB on each datacenter, so things even out and there's a consistent experience everywhere.
It shouldn't be a shock that this isn't actually happening. Welcome to chaos theory. Expecting people to behave as if no impediments exist when they do is wishful thinking at best.
The demand/cost in certain markets isn't high enough for people to jump servers, so the price differentials for items that sell 100+ times a day can be 2-3x, yes 200-300% different from server to server, meanwhile all the materials and time gated things people need to make those items are perpetually exhausted already, and it's been a few weeks?
Supply/demand doesn't fix this. The markets are concentrating on the few servers where items are priced high enough to justify emptying every single MB on a DC despite the massive rise in the cost of these materials. The steady state of this is that all players wishing to do crafting must relocate to one of those servers, and if game theory tells us anything, it's likely only one of those will win out in the end, making the entire datacenter a farm for just one server.
Interestingly, this result is the exact opposite of what Square Enix had intended to accomplish with this system. The fix is conceptually quite simple, even if technically difficult: have the datacenter actually share a single MB. If that's too big of an ask, MB access should be disabled to wanderers, so markets will remain stable on each server.