Question about the US Judicial System...?

The "definitive evidence" is at times quite subjective. If the government were able to simply convict without a trial, who determines what evidence is "definite"? Where will we draw the line between an automatic conviction and a fair and just trial? The best thing to do in order to ensure that our justice system is fair and legitimate is to give everyone the same opportunity to plead their case in front of a jury. If the evidence is really as definite as it seems, the jury will convict.