Because the reviewer is not magical. If there was something in the code the author couldn't see, the reviewer probably won't see it either.
The way to confirm that code does not have bugs is testing. So the reviewer is not looking at the code saying "this will work", they're looking at the code saying "I understand how this works, it makes sense."
Evidence that the code is safe is something that also should be provided in the PR, but it is not the main code. It is ideally test automation that is just as understandable as the feature code, but failing that ad-hoc test evidence or a specific step-by-step plan with evidence of execution is good too.
To have two primary purposes is difficult in English.
Because the reviewer is not magical. If there was something in the code the author couldn't see, the reviewer probably won't see it either.
The way to confirm that code does not have bugs is testing. So the reviewer is not looking at the code saying "this will work", they're looking at the code saying "I understand how this works, it makes sense."
Evidence that the code is safe is something that also should be provided in the PR, but it is not the main code. It is ideally test automation that is just as understandable as the feature code, but failing that ad-hoc test evidence or a specific step-by-step plan with evidence of execution is good too.