Did you ever see an engineered car that is made from parts of other various cars? Sure they exist...
People swap engines, and other parts but they always need to be modified to fit! However, while providing solution for specific need, lack of overall sound engineering does not maximize efficiency and ensures that this is the best possible designed car.
Boxing, Muay Thai, Full Contact Karate, Judo, all have techniques and training methods that were tested in the sports arena. But since each one of these fighting sports has specific unique rules excluding the other styles techniques to be used, they do not have an overall comprehensive view and approach to tackle the combinations of these sports.
Many Martial Artists that had encountered street experience realized that they are missing something and felt that their system was not complete. Some got stabbed, and some realized that they had to fight differently.
I remember my street fights prior to learning any martial art, where I had to rely on my instinct and logic. After I learned martial arts, I remember trying to apply a technique based on sparring experience.
However you must first acknowledge that self defense training should take into account that once another human being is trying to disregard your inalienable rights, there is a great chance you could lose your rights and your life as well in a confrontation.
It could take a split second for you to identify the problem and a split second for you to resolve it before it is too late. Do not forget, the perpetrator would like to use the element of surprise, the element of overpowering, over numbering, over armed etc…
In other sports Martial Arts, students learn to make their life easier with less pain by increasing their amount of fitness, endurance, resilience to pain, modifying their bone structure in their knuckles and shins, and nerve reaction by constantly training to take an impact.
This for example will have no bearing against a sharp knife which when used could give the user extension to his reach and maximum result at the tip of it with lesser effort of using force.
Instead of patchng up whole training systems like Judo Karate, Boxing and Jujitsu, or Aikido, etc, Pure Krav Maga looks at the principles of Motion, Tactical Approach timing and solutions. This drives the principles of striking, deflections, Kicking, Lunging, dynamic escape form holds and restraining attempts and quickly getting your opponent decommissioned.