I suspect there are a number of reasons. They need to be understood in the context of the criminal justice system's goals. It is not principally concerned with

I suspect there are a number of reasons. They need to be understood in the context of the criminal justice system's goals. It is not principally concerned with "doing justice" in any holistic political-philosophical sense. That is evident from its favoring of certain aspects of justice like punishment, retribution, and deterrence over things like procedural fairness and proportionality. That is to say its primary aim is punishing wrongdoers and making examples of them. Accordingly, any means or outcome which satisfies these goals is assumed to be "just". Any alternative-- even one in the cannon of "justice"-- is understood to be a failing or at best reckless. One reason this happens is that police and prosecutors alike put a lot of work into investigating crimes, and they do not like working in vain. They are especially bothered by the prospect of not apprehending anyone, and much more so by the prospect of being unable to convict someone they believe is guilty. It would be fair to say that they have at least some bias toward convicting someone-- anyone. It's also the case that they are sometimes wrong but very much don't want to believe they are. People easily find ways of justifying things they want to believe. Another reason is that trial lawyers tend to develop a preoccupation with winning. Consider that, for a prosecutor, the only way to win is to get a conviction. This can lead them down the path to all sorts of hell and is probably the biggest contributor to grand juries being shams and evidence being withheld. It is true that there are career benefits to winning a lot of cases, but for your average ADA that's typically just the privilege of being given more serious cases to prosecute. Plenty of ADA's lose a lot of trials with few ill consequences. Where this is more of a problem is with elected prosecutors, the actual District Attorneys. They are the ones who benefit most from high profile investigations, indictments, and convictions. It's very much in their political self-interest to get convictions. Lastly, the courts (judges) generally let prosecutors get away with all sorts of shenanigans for a variety of reasons. It may be that they themselves are "tough on crime", or prefer to give prosecutors the benefit of the doubt, or simply that they're lazy. Any trial lawyer will tell you it happens with shocking regularity that judges allow evidence into trial that they really shouldn't under the guise of the questionable rationale that they should just "let the jury decide." Let us not forget that one of Alexander Hamilton's main arguments in favor of the jury system was as a safeguard against corrupt judges. In short, little stops them from doing these things other than their own consciences, a handful of weakly observed procedural safeguards, and some extremely vigilant defense attorneys who fight back hard.