Because, to quote from The Big Short, “if we don’t give them the rating they want they’ll just walk down the street and go to [the other ratings agency].”

Does that make any sense? No.

In the case of “The Big Short” it did make sense, because the ratings were required by the government, not the purchasers (who often/usually disregarded the ratings for the purpose of valuation), and the sellers paid for the ratings.