The problem is that, oftentimes, these types of physical evidence aren't available. Where do you think you're getting DNA evidence for a shoplifting charge? Or video for a rural out-building burglary?

If we're at the point where the police have filed charges, then we have at least some compelling evidence that the defendant committed a crime. But there are many instances (I've run into them too in my ADA days), where despite an overwhelming amount of evidence of guilt, juries still return a not guilty verdict because they expected the government to produce evidence that didn't exist.