It's a technical limitation. The issue of having all the servers link back to one database is that it introduces an enormous bottleneck to the system. With tens of thousands people doing many things at once (e.g. opening their inventory, checking vendors, reading mail, moving zones, etc...) there are very few solutions available that can handle that transaction load, and they are all very very expensive. Splitting everything out between self enclosed servers lowers server costs, reduces system complexity, and simplifies maintenance. Actually it can be done, EvE can do 60k+ simultaneous users (and that's a much more robust market) but it's taken CCP eight years to get the game playable (arguably). That and they've spent the equivalent of a Ferrari on their database servers, good luck convincing an EA exec to green light an expense like that.