> Come to think of it, why is the criminal justice system not "tested"?
I can think of a few.
1. That costs time and money. Even if it would help, I see it being very difficult to justify, essentially, "secret shoppers" for the court system. I think it also kind of underestimates how much time, money, and effort goes into trials. It's not like a quick batch of unit tests. It involves lawyers, judges, jury selection (if it's a jury trial), etc. And juries themselves are made up of people who have to take time off work, miss important events, etc. You can't just (somehow while avoiding detection) fake a trial, and then go "surprise, this was just a test!" At the very least it would just piss a lot of people off.
That's not to say that there's no way to make sure our legal system is functioning properly, it's just I don't think mock trials are the right approach. Currently, I believe, the rest of the court system is relied upon to police itself, with the help of the legislative branch. Different levels of courts, ostensibly, keep each other in check (e.g. if you disagree with a lower court ruling, you can appeal to a higher court, which then makes a decision, and if you don't like that, you can appeal to the Supreme Court, etc.) Congress also has the power to do things like create federal judgeships, so presumably they have other powers to regulate and manage the federal judiciary (state judiciaries are, I imagine, left to the individual states to oversee.)
2. Cynically, there are certain vested interests that want the legal system to function the way it does (e.g. people who own private prisons and people who profit off of minor drug offenders being put away for life). There's also good old fashioned racism and classism, helping keep the overwhelmingly poor, black prison population stuck where they are.
3. I don't think it would actually tell you much; there are too many variables: different lawyers, different judges, different jurors, different facts, etc. Even if you ran the same trial with the exact same people and circumstances, but on a different day or something, you could get wildly different results. The court system is not deterministic, not by a long shot.
The court system also doesn't deal in absolutes like that. It's not "is this person definitely guilty of a crime?", it's "are we sure, beyond a reasonable doubt, that this person committed a crime (based on all of the evidence)?" Because it's impossible to know the entire truth. Sometimes it's impossible to know much of anything at all, at least with certainty. Did OJ commit those murders? We'll never /know/ know in the sense that when I flip a coin I can see that it lands on heads, or in the sense that I know the earth is a spheroid (i.e. it's something that happened in the past and we're trying to determine exactly the series of events given limited information).