Why do people search outside Aikido for IS?
Has a simple answer.
They haven't found what they are looking for in Aikido.
IMHO, the more I play with this stuff (IS), the more I see that its implied or indirectly taught in
Aikido and that most of us (okay, I) missed it (often even the ones thinking they are teaching it). I like the direct teaching and coming back and attempting to apply it.