It's not even as if the STORIES of FF were very good. I mean, Blyton had about four or five storylines and about a dozen stock archive characters, and just recycled them endlessly.
I meant to comment on this before. I agree that EB was obviously not 'high literature', but nevertheless, there must have been something to the stories that makes them still popular now, over 50 years after her death.
I'm not convinced that the storylines themselves are necessarily the important factor; I think it's often the settings, the context, the vibe, the familiarity, characters that children can relate to or wish they could have been like in whatever ways.
Look at soap operas nowadays. Many adults who watch one or two (but not all) of them don't usually flip back and forth between their regular one(s) and the one(s) they don't usually watch, depending on the storylines. They also don't look at what the storylines will be before deciding whether to watch any individual episode in isolation; for the vast majority, when it's the regular time for Coronation Street or Eastenders, they tune in and watch it - as they habitually do.
I think this was even more evident in some of Blyton's other works, such as the Adventure series (with Jack, Lucy-Ann, Philip and Dinah) - where large parts of it were just about the sense of having an adventure, being together, irrespective of what particular dramatic incidents occurred.