Because they didn’t exist at the time that book was written. I took a Unix/c class as a comp sci elective (yes, you read that right). However this was 1988 so no Linux. We worked mostly on pc’s which was pretty progressive for the time and mainframes. We had a small mini computer (can’t remember from who.. dec? That doesn’t sound right..). Anyway, forked processes, no threads. In fact, I didn’t know what a thread was until the windows nt announcement in the early 90s. The irony is evident.