If Valve has a monopoly, it is because their major competitors are a bunch of scumbags

"That said, the AO rating assignments were not a direct result of them being blockchain games, but rather how that was implemented.

“As is the case with all games, age rating assignments are dependent on the content of a game and the context in which it is presented to the player. The ESRB age and content rating system assesses whether products may reward real money, prizes with real world value, or something that can be exchanged for or converted to real money (i.e., gift cards, cryptocurrency, etc.), as well as whether there is cash spent to be eligible for such rewards. While it is not a blanket rule, it is possible that a blockchain-enabled game or app will warrant an Adults Only rating category assignment due to a combination of the factors discussed above.”

So I guess this probably covers most if not all “blockchain games” but it’s technically only the ones that are real money gambling.