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It's a metaphor, not an equivalence. It doesn't even have to be in the same field, as long as the meaning is conveyed. Bit rot is an excellent metaphor, even though it's nothing like physical rot if you get pedantic about it.

You're the first person I've heard say that the metaphor doesn't make any sense; meanwhile, a lot of people regularly use it, which implies that it makes sense to them. You're welcome to not like the metaphor, but you don't really get to declare that it's objectively wrong… it's a metaphor.

Software doesn't exist in isolation. It interacts with its host OS, shared libraries, other programs, protocols, file formats and external systems. If it can't keep up then it effectively rots even though not a single byte of it has changed.