A radical point of view. And as such it is of course wrong ;).

First of all, there are languages where dry-running your code with all parameters mocked is still a valid test run. Python, js, and Perl for instance make it very simple to have a stupid error in the routine that crashes every run.

But more importantly, a unit test usually executes inside the same process as the code. That gives you tremendous introspection capabilities and control over the execution flow. Testing for a specific path or scenario is exactly what you should do there.

Finally, what if not mocks, are in-memory filesystems or databases? They, too, won't show all the behaviors that the real thing will do. And so so test containers or even full dedicated environments. It's all going to be an approximation.