I think that the Horizon series is really interesting because it's actual level of popularity (pretty huge) doesn't quite line up with the way it is covered and discussed by the hardcore gaming communities and publications.
For example, which series do you think is more popular? Horizon or The Last of Us?
Based on how often The Last of Us is debated, discussed, rereleased, and praised, I would have thought it was way more popular than Horizon. However, it took The Last of Us Part II about two years to sell ten million copies, whereas Horizon Forbidden West did it in a year. Also, Horizon Zero Dawn has sold over 20 million copies since 2017, whereas the first Last of Us is only at around the same number (including both PS3 and PS4) since 2013.
Horizon is secretly a cash cow that a lot of people have played and love, but it's a cash cow that kind of just does its thing in the background while franchises like Assassins Creed, Cyberpunk, Last of Us, and Witcher soak up a lot more of the discourse and press. I'm not saying that's a bad thing, I just find it super interesting.