Orkhepaj: There is ongoing academic debate between ludology vs narratology approaches to defining a game.
then they are not games,
If you consider gameplay to be more important in games, you're taking the ludology approach. I consider story to be more important in games, so I take the narratology approach. Both camps will never resolve which approach has more precedent.
Regardless, VNs are games under a narratology approach. You may not think they're games, but to some academics and those who believe games can and do have narratives, they are. In most of them, you make interactive choices of some of your own volition that affect the flow of the story and the endings you get.
why not just watch an anime or read manga or just a book?
1. Animes usually come after VNs are made. And oftentimes, the anime don't have enough episodes to cover the contents of the entire VN. Best example is the Umineko anime, which cut a lot of its content and has its own original ending. Also, watching anime is a lot slower in terms of exchanging info compared to reading, which matters a lot if the VN is text dense. And as I've said, some VNs have many branching plotlines that are limited in how this is shown through anime (without time travel or parallel universes being involved).
2. You have to rely on scanlations until a publisher picks it up. And with print media, there are sometimes issues with being no longer in print, affecting supply. Also, there's no sound through the characters' voices or the background music. Umineko is another egregious example here. Also, the English publisher, Yen Press, opted not to use coloured text in its print, which is incredibly important to the narrative. And since it's out of print, we're looking at $100 USD / episode for eight episodes. So that's $800 USD to own it physically, which is inaccessible for many audiences.
3. Most Japanese books are not usually translated into English. Some VNs are not published in books. Therefore, it is difficult to get that same media in traditional book format. Books also don't have pictures, sprites, music, and usually don't have dialogue choices.