PC Gaming

That's a nonsense reason to ignore community creations. New players aren't drawn to games because of whether the community is making things; they're drawn because there are fun things in the game, regardless of where exactly they come from. That the community was allowed to contribute is not what drew so many people to Fortnite or Minecraft. Cosmetics make a lot of money, and mods can help with player retention as people get bored of vanilla, but they still need to be drawn in by the base game. That goes for Fortnite as much as it does League.

Besides, creators aren't generally drawn to making things for a game solely based on the tools available for doing so; they do it because they like the game. Even if that were the case, creators aren't a big group of people, nowhere near enough to move the needle on "having enough new players." That isn't part of the calculus Epic did when deciding support them, and it shouldn't be for Riot either.

Attempting to include community content doesn't put Riot in competition with other studios any more than they already are. Again, if they don't think their existing, massive community can make interesting content, that's one argument for not putting resources into it, but avoiding it because they think they'd have to draw people from other studios' communities is silly.