If reverse engineering is legal, why wouldn't this be? By reverse engineering you take a closed source program and gradually figure out how it works, eventually

If reverse engineering is legal, why wouldn't this be? By reverse engineering you take a closed source program and gradually figure out how it works, eventually ending up with source code that can be transformed into a binary that implements the same function. Reverse engineering would have to be illegal if what you say is true, because the source code you obtained from a copyrighted work is "tainted". There are also creative choices made in reverse engineering because there are many possible source codes that could compile to the same executable output. The author has to make countless choices around how to interpret assembly as higher level language constructs, and code organization in terms of functions and variables used. This isn't anything like zipping and unzipping some file, it's not a one to one transformation. As you can see we can absolutely have byte for byte identical output without having access to the original source code. I agree that it would be copyright infringement if you distribute the original game assets (sounds, fonts, textures, models), but I strongly disagree when it comes to the source code because of above factors. And clearly distributing reversed source code without those assets would not enable someone to even obtain the original binary. They'd need to already have them in the first place (e.g. by buying the game and dumping them). Also this is anecdotal but Nintendo is known for being quite belligerent with their copyright enforcement, they will take down various fan games in the name of protecting their IP. However to this day there are multiple full byte for byte decomps of Pokémon GBC games available on GitHub that haven't received so much as a DMCA takedown. Why wouldn't they care to take them down unless they couldn't do anything about reverse engineering? Nintendo still profits from reselling their old games after all (under their "Virtual Console" releases). The pattern repeats with Nintendo game console emulators, the only reason Nintendo is able to take some of them down is when these emulators include decryption keys for ROMs, they have never had a case for taking down an emulator on the basis of it behaving identically to the hardware they sell. Both in theory and in practice I fail to see why you would be right that game decomps are illegal or even in the gray area.