Sometimes I’ll have a typo in a title. Sometimes my keyboard doesn’t keep up with my typing. Of course I don’t notice this until after I post the thread. While we can edit the content of a post, we can’t edit the title.
Why?
I believe it’s just how vBulletin works.
Remember, you can report your own post now, so if you do mess up a thread title, just report the post and ask a mod to fix.