Question about epublished works

I read a lot on the Amazon Kindle and I have a Kindle unlimited account. I've noticed that, particularly, with erotic romance/erotica there seems to be a serial theme. By that I mean, lots of authors tend to write very short pieces strung out over several titles (Book 1, Book 2, ..., Book x), and there are few stand alone works.

I'm wondering why that is - is it some sort of formula in this particular genre?

Question about epublished works