How does the identification with these identities, of a separate I and the details of this I, come about? There are probably many answers to this, and one seems to be pure imitation.
The field finds itself functionally connected with a particular human self, and this human self lives with other human selves that the field takes itself to be. So it is just natural for it to similarly identify with this particular human self. It happens all around, so it must be the way to do it.
The necessity of identities
Identities is crucial for the functionality of this human self. It cannot function very well in the world without it. It has to be able to recognize itself as distinct from everything else, and as having a more fleshed out identity as well, including name, age, gender, and so on.
The optional sense of I
But the identification with these identities, the sense of I place on top of them, is not inevitable. The field of seeing and seen, of awake emptiness and form, can awaken to itself as a field, inherently absent of any I, and still be functionally connected with this human self. And the human self can still function with and from all these identities.
Growing up without a sense of I
Although the field almost automatically (it seems) nowadays identify with the identities of the human self it is connected with, it is probably not inevitable. The field can be awake to itself as a field, while functionally connected with a particular human self, that grows up, and develops and elaborates on various identities for itself, and the field does not need to be identified with these identities. They are a "me" and not an "I".