There are many things in this world which could not be seen through human eyes. The things which cannot be seen through or by humans make them the most afraid . The humans started colling the the "demon, asuras, etc.
So to deel with such things humans along with starting their civilizations also created sum methods and techniques to keep them selves safe from such creatures. When humans were only starting their civilizations many of the human population could still see these creatures as the wall between our world and their world was very thin that it could brake any time.
Humans after long time were eventually able to thicken this wall. with this came with a very high cost in as while trying to thicken the wall human civilization was about to be extinct many time.
Even many of the civilizations were even last to history with no evidence of them being real and were either termed as mithology, mith, falls story, or even forgotten.
So where were we.....
Ya let's continue from why we don't sea any of these arts anymore.
The arts which were created were so powerfull that it was believed that when a person could learn and master all the original 1000 techniques that person could easily become a god or even a entity grater than god's. The people who were able to learn about 900 of them were the people who were termed by the people as god's or avatar of goads. But this type of techniques could only be learned by a very few number of people. These people were either born with the ability to learn the following by birth or by a lot of traning with a lot of determination but still only few had a talant to do so.
but as we all know that every good thing always come to end .....