Well, editing makes things complicated from the technical point of view. So it's not a standard yet. Some clients have implemented it though. But you'd better not use it because people using other clients that don't support editing would see your pre-edited version of the post.

Besides, people take advantage of the editing feature. They post for something to appeal to people's emotional side. Get attentions and engagements. And then edit the post to make it something else. So it becomes misleading altogether. This is another reason why editing is not a standard feature yet.