Which is why so many emulators require you to somehow acquire the console's actual BIOS binary yourself (by manually dumping it from your own hardware, of course, and by no other means at all) and drop it into a file folder for the emulator to use.

The problem is, even if you write all the code yourself and require users to dump their own BIOSes and shit, when you get sued by Nintendo, you need the money to be able to hire people to defend yourself. You actually have to prove and most likely convince a judge you're right.