I really hate to do this because it sounds lazy, but I strongly encourage you to watch The Problem with Jon Stewart's episode on this. He explains it very well, and it's a lot of information, so if you are genuinely curious, I would highly recommend it. It will open your eyes to a level of criminal activity at the highest levels of our markets that will hopefully surprise you. It explains this, and more.
https://www.theproblem.com/episode-5-the-problem-with-the-st...