It comes from the publishers in the 90s. They needed an easy way to tell stores/distributors how popular they thought each of their games would be, to help them decide how many of a certain title the distributor should order. The games expected to be GotY contenders would be marked AAA, AA for otherwise decent games, A for more niche games and B for "this is a starshot, we're hoping it will sell enough to justify production costs". That then lead to more and more games being marked as AAA due to budgets getting increased, and the whole system became a bit redundant.