Gotcha, I did not know that
It's really hidden unless you know about how client/server applications work. The client side (what would be on the CD and/or the player's computer) is usually the smaller amount of code. It connects to and interacts with the server code. The server runs the world the client partly runs just your character, and not all of your character. If the client ran all of your character then you would be able to able to manipulate the files to 'hack' your character to gain advantages.
Think of the server as the DM in a D&D game and the character sheet is the client side. The DM keeps track of a whole lot more of the game than the clients do while also keeping track of the clients, too (so the players can't just erase and fudge a few stats).
(Before others who work with software development chime in with corrections, these are simple explanations. It really is a lot more complicated than that, but the complications aren't really important to the explanation).
And.. they have the various client discs at the office or in their own collections. There are some things they can derive from the client side, like reading what it's calling and back crawling from that. But, re-writing the server side completely from the client isn't that easy. I know that's how some of the emus started, so it is possible in some respects. But it's kinda like walking uphill, in the snow, both ways.
