Why do Muslims tend to act like they know everything about a book they haven't even read?

Some that I talk to act like they know the bible inside and out and spout off about how evil and corrupted it and its teachings are and then when I ask if they have ever actually read it they admit that they haven't. I just say that I don't follow the Koran and I don't claim to know everything about it because I don't. actually all I do know is that I personally don't think it came from God. I know that's all I can say.