We are Buddhas because the qualities of the Buddhahood are inherent within us. We have the potential to realize enlightenment. If we did not, then there would be no possibility of realizing anything close to enlightenment. We call this potential Buddha-nature.
When we say something like “we are Buddhas”, we’re speaking figuratively. It’s meant in the most fundamental sense and doesn’t mean that we have already attained anything and therefore there’s no need for practice, effort, or struggle. It just means that we have this potential, that the seed of Buddhahood is there.
But, in the end, it means attaining a state of mind where one sees that there is nothing to attain.
To Buddhas, there is no such thing as buddha. Buddha is just a concept, an idea, a mental representation that refers to the ability of a living being to realize something so subtle and deceptively simply that it defies adequate explanation. Nearly everything we think we know about buddhas belongs to the realm of appearance. Nagarjuna said that Buddhas “will stand outside appearance, outside sensation, outside concepts, outside forms, and outside consciousness.” We call it breaking free.
To a Buddha, concepts and forms and so on are unreal, empty. A Buddha has achieved this understanding by breaking free from his or her own mind:
The Buddha taught the dharma of quieting the mind . . . The primary point of the Thus-Gone One’s method consists of realizing the non-existence of the self. If the self does not exist, ego disappears. When the notions of a self and ego are purged, the mind is in the state of nirvana. Thus all living beings quiet their mind and break free. As soon as all living beings are calm in nirvana, there is no need to seek Buddhahood. Thus the mind that used to seek something is still, and all desire to grasp and to let go will vanish. Because the internal mind and external objects are empty, the One Mind remains immutable. This is the method to quiet the mind.
A Commentary on The Diamond Sutra by Ch’an Master Han Shan