Serial's third season podcast series covered this a bit, since the 1980's or so in the USA, we've decreased funding for the judicial system, and not increased prosecutors funding and staff to commensurate with increased crimes due to larger population (if crime rate stayed the same or even decreased.) This puts a lot of pressure on prosecutors to get plea bargains, the logical gaming of the system by prosecutors is to layer on as many of the hardest charges possible, then offer a plea bargain that has a fraction of the penalty of the initial charges. We do not have enough courthouses to actually let every defendent get judged by a jury of their peers. Even if one is innocent, rational people will choose to plea guilty given that prosecutors have something like a 95% conviction rate in the USA and all the resources of their local/state/federal government behind the prosecution.
Someone facetiously but based on my viewing of The Wire and The Shield, large city police departments are judged on finding someone to charge with reported crimes and having a lot of open unsolved crimes is seen as a terrible statistic, there's no extra reward in finding the actual person who did each crime so some might find a perverse incentive to clear crimes no matter what.
