@Ferris_vball I think it's mostly because of scientists' own ignorance or arrogance. Giving a good scientific talk is a skill. However you usually "pick up" this skill here and there and is generally assumed to known by students. In fact scientific communication is a scientific field of its own that needs to be thought to students so that at least people have an appreciation that giving a scientific talk takes more time and effort than putting few slides together. However, even in well meaning departments that mandate this kind of training or classes, it's usually seen as a formality and encumbrance by students and faculty. You wouldn't expect someone to be successful in your field without having learned the field. Yet scientific communication is treated similarly by scientists who have no formal training in it yet somehow "knows" how to give good talks.