I guess the latency between AWS and your data centre would have a negative impact on game performance.
That’s way your profit margins on the AWS servers is lower than self hosted ones but at least you’re making money.
Edit - And if a group of players raid a dungeon, the population of that dungeon is strictly limited, so you can park that raid on one server and don't worry at all about inter-player latency.
The bigger world is handled by slicing it up, but you still have a lot of communication going on with central databases for stuff like inventory management, chat, quests, etc. so you would probably try to keep all that within your own server racks.