There is a lot of software that isn’t an API, or the API is just a thin wrapper around the real implementation, or the API is one of the least important aspects

There is a lot of software that isn’t an API, or the API is just a thin wrapper around the real implementation, or the API is one of the least important aspects of the system from a customer perspective. For example, systems software is mostly about testing system behavior under various loads and resource constraints. The ability to even measure that reliably without the measurement interfering with the software operation is dependent on a lot of implementation detail.