Acharya Prashant explains that the ego exists because of one's identification with birth. As long as an individual considers themselves as someone who was born and will die, the ego will persist. He clarifies that the 'I' itself is the ego, and being a localized entity in time and space is the very definition of ego. This localization means that if one is in one place, they cannot be elsewhere, and if they are in the present, they cannot be in the past or future. This inherent limitation in time and space is the root cause of human suffering and the source of all desires.