Internal auditors are employees too, while they are not auditing they may suggest improvements like any other employee. But making the internal auditors responsible for suggesting improvements may cause employees to think it is not their job.
So an internal auditor, during an audit, sees an opportunity for improvement but says nothing about it in her report, but the next day makes the suggestion. What's the difference? If it's made clear to everyone in the company that it
is their job and fear of reporting has been removed, why would anyone think it
wasn't their job, regardless of what an auditor sees or doesn't see?
And it could distract the internal auditors from seeking evidence of effectiveness and reporting well-crafted nonconformity statements. I’d rather they did not suggest improvements while auditing because suggesting improvements is not auditing.
Writing "well-crafted nonconformity statements" isn't auditing either, for that matter. Auditing consists of observation and reporting what's been observed.