These days it is a bit more complicated. Now just about anyone sensible can become a "blessed" agency, a Nationally Recognized Statistical Rating Organization (NRSRO).
The catch is that the biggest rating agencies accept payments by the company issuing the bond. What happens is that Bank of Insanity gives Moody's a check for $500k to rate their super-senior diseased livestock bond. Moody's then says "at least 5% of the cows will probably survive, and $500k is a lot of money, so this bond is investment grade!"
The catch is that the biggest rating agencies accept payments by the company issuing the bond. What happens is that Bank of Insanity gives Moody's a check for $500k to rate their super-senior diseased livestock bond. Moody's then says "at least 5% of the cows will probably survive, and $500k is a lot of money, so this bond is investment grade!"
That's why things work out so poorly.