I've been to several seminar talks given by historians, and the norm seems to be to read your paper, perhaps with an occasional comment added in the middle. Usually even visual aids are not used. This seems to make for a boring and hard-to-follow talk at least for non-historians like myself, and after such talks I often hear other non-historians complaining about this format. This does not seem to be the norm in other fields, and I think this would be quite frowned upon if done in many scientific communities, although, of course, a scientific paper is very different from an historical one. Occasionally, there is a talk where the historian is very passionate and does not read off of a paper, I tend to find these much more compelling. So why do historians (and perhaps others in the humanities) prefer the read-your-paper-aloud method?
This does not seem to be the norm in other fields
I'm not sure what you mean.
At professional conferences, everyone - in all fields - reads off or has access to of a prepared text. They are attempting to relate complicated and long-pondered ideas, not talk off the top of their head for 20 minutes. Precision of speech and thought is vital.
There is, of course, no uniformity in what kind of paper a person would read off - if you're reading a paper that you would submit to a journal, and not a prepared talk, you're doing it wrong in any field. Moreover, the delivery of a good talk is a skill all of its own requiring practice, something that far too few programs train their PhD students in.
Visual aids should only be used when absolutely necessary, which, for most history talks, they aren't.
Seminar talks aren't TED nonsense. They exist to inform an intellectual audience and to obtain useful feedback for future publications.
Try standing up and talking for 20 minutes to an hour in front of a specialist audience which will definitely be judging not only your style but your content.
The idea, in my opinion, in presenting research at a research seminar is feedback from a community wider than your own intimate circle. Thus you want your audience to have as clear an understanding as you can offer of the ideas. You can also expect a level of familiarity with jargon, ideas, concepts, and thinkers particular to your field (unlike a conference this is a much more self-selecting audience). You will be expecting criticism of methods, scope, and your thesis - why risk opening yourself up to further attack or wasting valuable time for discussion clarifying points you could have made were you not meandering through an irrelevant PowerPoint presentation, or Ummming and Ahhhing?
Research seminars, while welcoming the public and students, are not directly aimed at either demographic. They are part of a longstanding tradition of academic communication. At one seminar in the IHR (Senate House, London) there was a rule that no-one was to clap after the speaker had finished delivering their work - as it created an informal hierarchy between audience and speaker.
Not perhaps to sound rude, but in one sense, you as a non-historian aren't actually the intended audience for the seminar.
To back up TheGreenReaper7, explaining what can be complex and detailed work and ideas for relatively long periods of time isn't exactly something you do off the top of your head or without notes (unless you have an amazing memory or know the topic incredibly well).
The role of the academic seminar isn't simply a lecture or presentation to anyone, but in presenting your work and ideas to colleagues, in order to gain feedback from other historians on your work.
This is odd; usually all the talks I go to will have some kind of visual aid-even if there's no explicitly visual component like one would expect with an art history paper people will have a slideshow with related visuals and quotations from the primary texts that are being referred to.