It's not the engine that needs to be reverse engineered. It's the balance and legitimacy. If you offload all the RNG and cause equipment to be controlled by a central entity, then it becomes both opaque and a single-point-of-failure. People know EVERYTHING about Diablo 2 drops, because you have the machine logic right there. Not true about, say, World of Warcraft, where people know a lot of macro statistics about drop rates and so on through crowd-sourced data gathering, but they don't know a lot of things, like boss pathing rules and event scripting.
Running a private server for a MMO has a ton of fundamental barriers. 1) Players don't get timely balance updates. 2) Players have no confidence that admins won't hack/give away items/cause crazy events etc. (Central point of authority = central point of corruption. Even if there don't happen to be any game-breakingly generous admins on a server, you can't signal that reliably to potential players.) 3) Servers may disappear. Since info is central, but private servers may fold under takedown notices, owners running out of money, technical problems, lack of other players, or anything else, players can't have trust that their time spent will have lasting rewards. Credibility is almost impossible to build.
Basically, by making games reliant on a central authority, you force the element of trust and credibility into the picture. Usually the only server with any credibility is the one run by the game company itself, since ideally you can trust that their actions will be in the best long-term interests of their game and audience. (EA in particular lost that trust a long time ago, I haven't bought a game of theirs in a LONG time. But Valve, or Blizzard, and even Ubisoft to a lesser extent still have that credibility.)