The problem is that there's an adversary who, in practice, is not attempting to lead the jury to the correct conclusion, but lead them to a conviction. And that

The problem is that there's an adversary who, in practice, is not attempting to lead the jury to the correct conclusion, but lead them to a conviction. And that adversary will be equally able to explain in an understandable way why, despite your explanation to the contrary, you in fact are guilty of the charges. The system works when juries are able to differentiate between expert witnesses, and that may very well require more expertise than just applying everyday common knowledge to two competing expositions.