why we're not all buddhas

Buddha nature is the nature of every sentient being in the universe. We hear this basic tenet a lot in Buddhist teachings, but described in a certain way it hits home very clearly for me. Tulku Urgyen Rinpoché, in a chapter of Quintessential Dzogchen entitled Shamatha & Vipashyana, points out as usual that there is not a tiny speck of difference, be it in size or in colour or in strength, between the buddha nature of a microscopic insect and the buddha nature of a fully enlightened buddha. Rinpoché then answers the question of the difference between the two: while a buddha recognizes his or her emptiness nature, the cognizant aspect of a sentient being's mind grasps onto phenomena -- including his or her own mind -- as real (p. 158). We all experience worldly phenomena in spite of its inherent emptiness, but we non-buddhas, in spite of our mind's aspect of emptiness, grasp onto phenomena with our mind's aspect of cognizance. The buddha's mind is also comprised of the two aspects of emptiness and cognizance, but without cognizance grasping like this.