They are grandfathered in as an oligopoly because by law many large funds are prohibited from investing in securities below a certain credit rating. Additional

They are grandfathered in as an oligopoly because by law many large funds are prohibited from investing in securities below a certain credit rating. Additionally, if you are on the other end (the one getting the rating), you pay the credit agencies for a rating so that buyers of debt will buy it. It's basically required that you have a rating if you want to issue debt. It's a racket in some ways with a ton of bad incentives and inefficiency. Kind of like a lot of things.