Actually, prosecutors are governed by many rules from constitutional and statutory law as well as canons of ethics that apply to their special position. They have an obligation to promptly disclose any and all evidence they encounter favorable to the defendant. They have an obligation to drop charges at any point that it is apparent that there is no probable cause that the defendant is guilty of a crime, and not to prosecute when it is clear that the state cannot meet its burden of proof (in a criminal matter, beyond a reasonable doubt) by admissible evidence.
That being said, prosecutors are humans, and often-times egotistical, political, cynical, and more than occasionally vindictive humans with all that entails. So, it is hardly uncommon to see rules already stacked in the state's favor bent or broken.
Trials are virtually extinct in the USA. With few exceptions, if you find yourself going to trial, you are not in a good place. While good lawyers should be trial lawyers and unafraid of trying any triable case, the vast majority of the time if you are getting off, you are getting off pretrial with a dismissal or deferred prosecution agreement. If you lose on pretrial motions and the state is intent on prosecuting, then most of the time if your attorney can get you a deal and recommends that you take it, then it's a choice between pride/"truth" and rolling dice loaded against you and paying for it big time.
While in principle as a defendant you are not supposed to be punished for taking your right to go to trial, the plea system punishes you heavily, and if you refuse to "take responsibility" by admitting guilty (the earlier the better) and expressing remorse, then you get punished even more post-conviction, with a heavier sentence, and then certainly being denied parole if that is available in the state system.