From Kotaku's Jason Schreier: https://kotaku.com/esrb-says-it-doesnt-see-loot-boxes-as-gambling-1819363091
ESRB does not consider loot boxes to be gambling, said an ESRB spokesperson in an e-mail. While theres an element of chance in these mechanics, the player is always guaranteed to receive in-game content (even if the player unfortunately receives something they dont want). We think of it as a similar principle to collectible card games: Sometimes youll open a pack and get a brand new holographic card youve had your eye on for a while. But other times youll end up with a pack of cards you already have.
According to the ESRBs criteria, Real Gambling is any sort of wagering involving real cash, while Simulated Gambling means that the player can gamble without betting or wagering real cash or currency. The spokesperson added that any game with real gambling will always receive an Adults Only rating, which would be poisonous for big publishers, as most big-box retailers will not sell A-O games in their stores.