A grand jury hears just one side of the story. Prosecutors use a GJ when they don't have solid proof of wrongdoing, but just think that there's something going on.
I don't mean to sound snarky, but: that's because they hire people who know how to merge linked lists or reverse words in a string or (insert interview problem du-jour) ; they don't hire people who can really solve real-world problems.
I may sound old when I say this, but my impression of interviewing there was that they're just a bunch of kids who like working on 'cool' or new things, and don't really care much about the real-world impact.
No, this is not sour grapes; I was recruited for my skillset, but was really disappointed to be tested on bullshit problems; I pointed out specific issues with their site that needed fixing and how they could be fixed, but it didn't really matter to them.