It's because in software engineering your work product is confidential/proprietary and work samples take a very long time to produce. If you could review someone's code from their previous jobs, I think it would be quite easy to vet applicants. And it would be much easier to move from job to job. Instead we have to simulate work samples through whiteboard coding, pair programming sessions, take home coding projects,…
This helps us look for good signal around their communication skills, mentoring skills, empathy, reasoning skills and software development skills.
We used this review as a diagnostic - there are no wrong answers since we know they’re not aware of our style guide and conventions.
It worked for us. Maybe it could work for others.