Threads do not "belong" to the OP. A user here has control over the content of his or her own posts, but not the thread in which they are posted. And that includes OPs. While users here frequently refer to "my thread," it's really a misnomer. It would be more correct to say "my OP" or "the thread I created." Don't let the colloqualism fool you into a sense of control. You cannot lock threads that you create for the same reason that you cannot edit other users' posts.