Correct. The backends to these online games isn’t complicated, but it is protected. They want the ability to resell the entire service to another game studio to run.
This has happened a lot in the past. EverQuest, Pirates, Lots of mmos have changed studios and with that, the backend services needed to run them.
Now, that said, there are a few countries in the EU that you could reverse engineer the server and it’s totally legal. Some of the best fun I’ve had were on private WoW or Lineage 2 servers.
It is all about IP, and like Hollywood nowadays, how to repackaged it in remakes and emulation.
A bit hard if we're allowed to just play the original versions.