AI doesn’t forget because it is not capable of remembering because it is a machine and because it does not (yet) exist and it may never. Only of a thing capable of forming and keeping memories can we say that it has lost them. A machine cannot remember anymore than a brain can remember. Brains don’t remember, people remember. A (mostly) fully functioning brain and nervous system along with its concomitant array of sensory apparatus (sense organs) is needed to remember anything. Without senses there is nothing to remember for there is nothing to experience. Experiences lay the foundations of memory. They pour the concrete if you will. The structure that is built on top needs that foundation to stand but it can take many different shapes or none at all.
AI, being a machine, cannot remember, therefore it cannot forget. Of course.what is called AI today is what was called deep learning, or systems of intelligence, or special intelligence yesterday. Because a true AI has failed to materialize the proponents of such things have conveniently moved the goalposts.