Does one think that a knife fight in real life would be easy? Anyone with even a basic level of martial arts training knows that if a knife comes out, someone is getting cut. I would say that most people instinctually know that if someone pulls a knife, this is a situation they want out of.

But to answer your question, many martial artists train with weapons. It's fairly easy to use a dummy knife to simulate real combat without harming anyone. And many fighters throughout history would have trained with weapons to simulate real combat. I cannot speak for these knife techniques in particular because that was never my specialty, but there are plenty of legitimate techniques out there. In terms of what they are teaching in self defense classes, I find that a very mixed bag. Some focus strictly on tricks that seem cool to the untrained, but are hard to pull off in real life without a great deal of practice, while some instructors take a more realistic approach (be aware, have a plan, use a minimum of physical technique to extricate yourself as quickly as possible)