I don't understand. There's no competition to be had in this space. The people who play your game are the ones who'd be generating the content; those who make stuff for Minecraft or whatever can't be competed over because they already don't play League.
I don't know what alternate reality these people live in where offering their players the opportunity to contribute is some secret sauce that would put them in direct competition with other tech giants.
There's little business sense to make it exclusively for the current player base. You'd be risking wringing your customers dry. It HAS to attract new players and thus new income sources. If they can't compete, then it's not worth the time and money to create and maintain those tools. You compete with other companies in a space purely by investing your time and money in that space because anything spent is expected to eventually turn a profit.
If they don't think their community would create items that other players want to buy, that's a different thing. The players and creators are already invested in their game; they have a playerbase of millions. Hand picking a few community created things to resell to their customers in the same vein as Valve with CS2 and TF2, or Epic with Fortnite, doesn't make them competitors any more than they already are.