My recollection is that the original thought process way back when was that people come to the forum to share, learn, teach, and discuss. Sharing of images in public (once that ability was introduced) was to be part of all that. The sharing of images privately could, and can still, be accomplished in numerous ways outside of the forum. Additionally, whereas the admins and moderators don't have permissions to read or moderate what is said in the private messages, having the ability to attach images of any type of content is just another risk to the friendly, open, sharing culture that has enabled this forum to survive when so many others similar have disappeared.
Peter