To wonder why we cannot "perceive perception" is a fallacy based on a linguistic misunderstanding: it is like asking why we cannot paint red red, or eat eating. Perception is not an event in the world, or a relation between two objects; it is a transcendent, and it is what constitutes the world. It is perception which gives the world its form and shape, and what makes objects what they are (i.e. gives them their object-nature).
This is why perception can never happen within the world. To perceive someone preceiving would place their world, their selfhood, entirely within mine - and they would become a function of myself, an object of mine rather than a subject in their own right. They would be a figment of my imagination.
The corollary of this: although the self is transcendent, it does not transcend other selves. Selfhood itself is transcendent. It can never be contained in a world for it is the condition of a world, the canvas on which a world is drawn. Selfhood is the boundary condition of a world, and likewise we may understand God as the boundary condition of the world (I speak metaphorically - this is not supposed to be taken literally or mapped in any way on to reality). Just as 'I' cannot be proved (subjectivity cannot be established within the world for it is only that by-which a world exists), God too cannot have a presence within the world, as it is the limit, the boundary by which world-ness is defined. God is never object but only subject, the subject of subjects, the boundary condition by which subjectivity is possible.
As such it is undefinable, it is that by-which, i.e. Being. It is the sine-qua-non of life.