Most games have all the assets loaded onto every client, and they just synchronize over the server like a glorified IRC channel. People reverse-engineered WoW

Most games have all the assets loaded onto every client, and they just synchronize over the server like a glorified IRC channel. People reverse-engineered WoW servers, and then ran their own WoW communities for a decade until Blizzard finally decided to get back into the game. (but then Blizzard re-implemented the old content into the new engine, rather than starting up the old server code, so ended up being a lot more effort for them than just flipping a switch)