So what’s fun about that series has changed overtime.
When I first got l4d1 I was excited to share a spooky experience with friends. I found the atmosphere of the campaigns and characters interaction to be enjoyable.
When l4d2 came out I was a seasoned player and I felt that the level design was worse (but still fun) than the first game except for Hard Rain, that campaign was very challenging. However the addition of melee weapons was huge and the new variety of monsters appreciated.
But what kept me coming back was how much love went into the game over the course of its lifetime. All the l4d1 campaigns and characters got imported, extra campaigns were added. All for free (on PC) to enjoy.
The workshop added infinite replayability as there’s so many custom campaigns and other whacky content to explore.
Also the PvP mode where your team takes turns doing the campaign and playing as the zombies was easily the best way to play the game in my opinion.
So if you didn’t get into the game with a group of friends back when it was the zombie game to play. I could see why you might not think it’s fun or if you didn’t have any friends to play with. It’s not a great single player experience and the AI sucks.