It seems like it would be so much more productive to give the candidate a writing prompt where they can show their skills, instead of such a poor and generic ex

It seems like it would be so much more productive to give the candidate a writing prompt where they can show their skills, instead of such a poor and generic exercise of “tell me who you are and why you want this job even though you’ve spoken to no one at the company yet”. If you value good spec writing then give them a spec-writing exercise.