There are several reasons...
When the first post or a thread is deleted, it removes the entire thread and so one members action impacts any who have responded. For the same reason the edit period limited. If a post is substantially altered after others have responded the context may change and make those responses look odd. That said, we understand that sometimes an edit is needed and so if you need an assist, please let a moderator know which post and we can help out.
“You have brains in your head. You have feet in your shoes. You can steer yourself any direction you choose.” -- Dr. Seuss