Why is it knife defense instructors never show a opponent charging at the victim and slashing or stabbing repeatedly -its only a static swing or thrust and the victim practices taking the knife away ?
Never showing it or never showing it on Youtube? I've seen lots of people teach defenses against attacks other than static swings and thrusts. I don't watch martial arts on Youtube so I don't know what it shown there.
That said, there are some reasons people could do this. One is that in martial arts there is often a tendency to square off with an opponent.
Another is that in order to learn how to use the defenses in a dynamic scenario you'll need to have the mechanics down first. It is possible that the instructors are not showing their entire courses but maybe some mechanics or drills. I've seen people take things like this out of context and tell others that the drills were what the instructor taught as self-defense.
Another possibility, one that there is no excuse for after the early 1990s when training videos became common, is that some instructors don't know anything about knives. Some don't even want to know, thinking such training is immoral. The problem is that without knowing realistic knife use they'll only have unrealistic attacks.
I can see how those stuck in the "I only practice what my sensei teaches" or "I only practice what I need for my next rank" types can also fall into this problem; the ones like this I know often have a very limited range of skill and knowledge even if they are good at what they do know.
