Because it may be the nominative case of personal pronouns; It may also be the personal pronoun accusative.

Analysis:

It: pron. It ... The nominative case (the subject in the sentence) or the objective case (the object in the sentence).

She told me how to do it. (accusative, as the object)

She told me how to do it.

I have looked it up in the encyclopedia.

I have looked it up in the encyclopedia.

This is a luxury for me.

This is a luxury for me.

They are all there except me.

They are all there except me.

Let them do it at once. (objective situation, as the object)

Let them do it at once.

I let them out lest I should be both. (objective situation, as the object)

I let them out lest they disturb me.