They are trying to avoid a situation where you end up with one juror who watches a lot of CSI and insists that they need forensic evidence to convict, despite having a dozen eye-witnesses. If a juror cannot imagine a circumstance where the evidence could be beyond a reasonable doubt based on non-forensic evidence, then they aren't suitable to be a juror.
For example, if you're sitting in your living room with a bunch of other people, many of whom know each other, and two people start fighting, you are all witnessing a crime and you can also all identify the two people fighting. It would be ridiculous to require DNA evidence in that situation.