For most prosecutors, it is a game which determines the monetary and power value of their careers.  So, rules are bent, exculpatory evidence is suppressed, and technicalities twist the concept of truth into knots.

Judges are human and have their own biases.  If they are not challenged about their biases, they will continue using them in their decisions.

Hence, a defendant must have competent representation in order to have a fair chance at acquittal.