It's democracy. The purpose of a jury trial isn't to get the most accurate result, it's to give the people direct control over the government. It's just as true, if not more true, to say that elections are a terrible way of fairly selecting lawmakers and presidents.
The problem there is that correctness is almost completely on par with justice. If the jury gets it wrong (condemning an innocent man, what have you) law be damned, justice was not served.
The policies of politicos is a much more grey area with consequences not so immediate and very subjective many, many years after the fact.
The tradeoff is between a possibly marginal improvement in accuracy between juries and trained judges and the risk of the entire system becoming unaccountable and systemically unjust. Since the system is already designed to optimize for false acquittals, it's not an entirely bad idea.
