There was a Vox article on this recent-ish phenomenon a few years ago. Unfortunately, Vox is no longer free and I’m not a subscriber, so I can’t post it. If I remember correctly, the gist was that “a novel” is a useful tag for those who uncover a book in a less organized setting than a physical bookshop (where you’d think, “of course it’s a novel— it’s under Fiction!”). This could be in an online marketplace or by using a search algorithm that might not just uncover one kind of book, so might return book fiction and nonfiction.
Separately, it also became a bit of a trend in literary fiction in the last decade or so for reasons I don’t know. Maybe the same reason? Maybe it sounds erudite or “classic” in some way?