Why? my professor asked.
Because you don't listen to the lawyers or the judge, he was told. "Big brain" people try to play Matlock and solve the case themselves, or invent theories in their imagination, or get into absurd arguments in the jury room. They screw up the whole process that's designed to bring a high level of rigour and fairness to the situation--what should and shouldn't be considered, how the law applies, etc. They're wild cards that turn trials into a game of chance instead of an orderly adversarial confrontation that's heavily regulated.
People mock jurors routinely, especially the ones who explain their reasoning in interviews afterwards. But studies routinely find that judges agree with the jury's verdict about 80% of the time.
But here's the thing: their decisions, ideally, should be made on the basis of the evidence presented to them, their judgement of the credibility of the witnesses, and the instructions of the judge in how to map the facts as they determine them, to the law as described to them.
We shouldn't be okay with a juror voting guilty because of the defendent's race; we shouldn't be okay with someone who says "I'm just going to flip a coin"; I'm not sure it's a bad thing to exclude people who, in the experience of the lawyers trying the case, don't pay attention and invent their own rationales out of whole cloth and make an irrational decision by the standards of the legal reasoning the trial is supposed to provide.
Except for cops. Cops certainly like to have fellow cops on their juries. There is an old joke about removing Manhattan jury cases to Staten Island. All the NYPD cops seem to retire there. So if the case involves potential police wrongdoing or even just police testimony, one side wants a Staten Island jury as many jurors will have at least some familial connection to the police.