[NSFW?] Exclusive FWB friends with benefits so why are they not dating?

[NSFW] This is an interesting question posed by a young woman. She says her and a good friend are FWB, she only boinks him, and he only boinks her. They are free to seek other partners to date, but they never seem to get around to it. She wonders if they are boinking each other exclusively and no one else, why are they not dating? I believe I know the answer she seeks but I am curious of the mind the Lagoon has of this. [NSFW]