[imtringued]

The real reason is that open source software completely eliminates the need for reverse engineering. RE only makes sense when you don't have access to the source code. Doing things like private server emulation for an MMORPG might sound cool but the reality is that half the game content exists server side. You're not able to invest enough resources to rebuild the original experience and even if you could why not spend those resources on your own game? The demand for reverse engineering is pretty small.

[mysterydip]

There's many MMOs that now officially only exist as memory, either shut down or altered in such a way that they aren't the same games anymore. Some people don't want to make their own game, they want to play the game they used to with friends/family years ago that is now unavailable. The amount of community effort put into some private servers is impressive. I wouldn't spend my own time doing it, but I'm glad there are people that do.

[cycloptic]

I just find that more disappointing because the copyright holder is positioned to be able to open-source or public-domain-dedicate the material but they don't for various reasons and it rots on a backup drive somewhere or gets lost, and now unnecessary duplication of effort must occur to recreate it.

Often it seems rather paradoxical in nature because the main reason they don't open-source it is that they're waiting for a time in the future when the demand rises and it will be worth something again, and yet the increased demand only happens because of the efforts of reverse engineers keeping the community alive. It's almost always impossible for the reverse engineers to legally get paid for this work too. The only hopes for that is either for the copyright holders to raise enough money and decide to hire them, or to do an anonymous patreon and hope it doesn't attract the wrong kind of attention.