Even within a single computer or program, there is need for communication protocols and shared understanding - such as types, data schema, function signatures. It's the interface between functions, programs, languages, machines.

It could also be argued that "reuse" doesn't necessarily mean reusing the actual code as material, but reusing the concepts and algorithms. In that sense, most code is reuse of some previous code, written differently every time but expressing the same ideas, building on prior art and history.

That might support GP's comment that "code reuse" is overemphasized, since the code itself is not what's valuable, what the user wants is the computation it represents. If you can speak to a computer and get the same result, then no code is even necessary as a medium. (But internally, code is being generated on the fly.)

I think we shouldn't get too hung up on specific artifacts.

The point is that specifying and verifying requirements is a lot of work. It takes time and resources. This work has to be reused somehow.

We haven't found a way to precisely specify and verify requirements using only natural language. It requires formal language. Formal language that can be used by machines is called code.

So this is what leads me to the conclusion that we need some form of code reuse. But if we do have formal specifications, implementations can change and do not necessarily have to be reused. The question is why not.