I don't think there is just one problem, although that is a big part of it. Prosecutors and judges are often picked by how profitable they are to the local courts, neither cops nor prosecutors lose anything if they make up and push bullshit charges even when caught red handed, states and locales continuously tack on all sorts of random court charges to increase revenues, and there is no verification that any "programs" they tack on and receive kickbacks for actually improve anyone's situation. Not to mention all the inter-family connections and "ill scratch your back if you scratch mine" backroom dealings between cops, courts, prosecutors, and jails.

The only checks on any part of the justice system are from within the justice system itself, and its pretty hard to get those people to voluntarily cut their own pay, staff, benefits, and control in the name of justice, especially when it would most likely just get them fired and blacklisted. There are very little checks against what they do, and the people who have been involved with the courts and know how it works either benefit personally from it, or are considered a no-good criminal who's word is given zero heed. In most places if you get a felony you even lose your right to vote.