Think of every other job in the world where people cut corners and slack and get corrupt. Everyone from the guy who has to flip burgers to brain surgeons. Now imagine them with the ability to completely screw up your life at their whim, from arrest to prosecution to judging.

Law enforcement is a massive, for-profit industry. Everyone is out to improve their arrest/prosecution record. If you can't afford to defend yourself, you are an easy target. Even if you win, you still lose for having to go through the process.