> What is the logic here?

It is right often enough that your time is better spent testing the functionality than the code.

Sometimes it’s not right, and you need to re-instruct (often) or dive in (not very often).

I can’t imagine retesting all the functionality of a well established product for possible regressions not being stupidly time consuming. This is the very reason why we have unit tests in the first place, and why they are far more numerous in tests than end-to-end ones.