Early access game that is not made to have FOMO or player retention. The game will eventually have a definitive end and most likely won't have any reason to continue playing after completing the story, at least outside of "because you wanted to."

No fear of missing out or forced player retention with things only obtainable for limited windows that you'll miss out on forever if you don't play is why the game will dip in numbers when there has not been a major update and inverse; it will spike when new content has been added. Games as a service are the only ones that really matter with player count, MMO's or PVP games. Battle passes and season exclusives are the most common forced player retention models, which pray on the users fear of missing out to get them to feel obligated to play or else miss out on the "thing."

Besides, player count means nothing for private peer to peer multiplayer games. It matters not how many random people around the world are playing, only the ones you're in contact with to play together with.