What separates the “fast food” books from the truly good books?

I’ve seen a lot of discussions about the books made popular via booktok in other subreddits. Books like Fourth Wing, From Blood and Ash, etc. The general consensus seems to be that most reader admit they are not technically good, but they are enjoyable. They aren’t books that will stick with them long term. I’ve seen the term “Fast food books” thrown around on multiple occasions.

I’m curious what everyone thinks is the separating factor between these books, and ones that you consider truly good. Is it just one missing ingredient? Or is it a combination of factors to you?

For me, I feel like they’re missing a theme. That message that the author wants to get across to you. Sure, they might write an enjoyable plot, have believable character, and technically good writing, but without that theme, I’m not going to be still thinking about the book years later.

I’d love to hear your thoughts!

Edit: I’m not in any way trying today that these books are bad. Reading is reading, and there is nothing wrong with a book that is enjoyable and easy to read. I have just seen multiple posts where people refer to certain books as the reading equivalent of fast food. Enjoyable and easy to read.