I was gone for several months, and others have been gone from time to time as well. You never know when someone will come back. And if the admins/mods took actions to manually delete and restore accounts/posts (say when someone returned), it would make more work for them. Besides, if they were active in some accounts, but left for a time, for whatever reason; deleting all posts from them from that given thread (if one were to delete the account cleanly), would also seriously break up the flow of the conv...
An unused account, probably doesn't take that much space on the servers, unless it's chock full of file attachments and what not...