I think the idea is to test (with the jury as a sample) if the evidence is compelling enough for society at large to agree with the verdict. It's rarely going to be 100% clear cut so the jury has to use their judgment.

Or perhaps the standards of evidence established for a pre-industrial society when eye-witness testimony was the best that could reasonably be achieved is not necessarily the optimal system for a digital society where everyone carries a high definition video camera in their pocket at all times.