Juries are meant to bring the perspective of ordinary citizens into the justice process, not to function as pseudo-legal professionals. If jurors were trained in law, they could become less representative of the community and more influenced by a legalistic way of thinking which shifts decision-making away from shared community judgment and toward strict legal interpretations. It’s considered a good thing that juries are not legal professionals, and have no more understanding of the law than any average person in the community.

The logical extension of the OP question is that all juries be composed entirely of people with law degrees. This would undermine the purpose of juries in our judicial system.