My guess is that they want them as a way to try to own the user, to make them have the "app store owner" role and have users go through them to get stuff done. Otherwise, if users were just using tools that used OpenAI behind the scenes, they're more vulnerable to the makers of those tools swapping vendors.
However... that results in them owning the user experience and the responsibility for keeping the user from being surprised in a bad way. The complaint from the user here was framed as being a GPT-4 problem, not a plugin problem, in a way that exposes OpenAI directly to more frustration than if they were interacting directly with someone else's product.