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.