Not even “competition”. BGA is more or less allowed because it’s assumed it drives sales. A Quoridor-only site would probably sell Quoridor fine, but BGA will also sell Cuarto, or a dozen other Gigamic games. Or vice versa - BGA probably does more to draw e.g. Catan players into checking out Quoridor than a dedicated Quoridor site would.
Also BGA explicitly asks for permission to post these games.
Also BGA is pretty big and large. It's already one of the best places to put digital implementations of games, and that's why you see major hobby games come out on there, like Wingspan and Ark Nova, with about 75 or more new games added to the site every year, including games that haven't even been released yet and still on Kickstarter, like Undergrove recently.
And yeah, I've bought probably at least a dozen games now after playing them on BGA first. It's fun to play them on BGA, but I'd also like to play them in person with my friends, too.
