1. It's not you. You're showing an audience another presenter, maybe even in voice-over, who is potentially very different from you and your style.
2. It's not your message. However close a video is to your ideas, it's probably not identical, since you didn't produce it.
3. It's not yours. The copyright may be owned by someone else. They may never find out, but it's still illegal to use it if you don't have permission.
4. It's not fair. I've seen presenters fill up to half of their speeches with video material. That's hardly fair to an organiser that's paying you for your stuff.
5. It's not in flow. Using a video can break up the flow of your speech, and distract your audience.
6. It's not new. Many viral videos have been seen by many people. The first viewing may induce a gasp. Subsequent viewings my induce a groan.
7. It's not good enough. Your material is superb, so don't dilute it with someone else's second-class stuff.
Of course, there are good reasons to use video too - what would you suggest they are? Or do you agree with the points above?