The fact that code is all there and deterministic isn't sufficient. People use newer versions of compilers and you can't predict what the people who write compilers will do (people are black boxes). The compilers may do something you don't expect but still conform to the spec.
The point is that there is no one implementation of C. And the same is true for any other language too (to a lesser extent).
Unless you're coding against very specific versions of OS and compiler and runtime environment, you don't have code to inspect. You are not inspecting all the underlying dependencies when you write your code. You just assume they work.
It doesn't matter whether they are black boxes if you don't routinely inspect the boxes. Perhaps you do, but for most people they don't. So it doesn't matter as much.