One of the best things about a forum is that is has depth and history; you can search this. No one does, but you can.
If we were to delete those members who haven't logged on for <<a period of time>> we'd be messing with the flow of threads, subscriptions etc. There is no way we would do this.
Also, I have coined the phrase "forum fatigue". Every now and again folks just have something else to do for a while, or need a break and so they become inactive for a period.