We often hear the phrase, "look who it is" or like the one you mentioned. I think it is used that way to mean that there could possibly be anyone on the other side of the door. Hence it is generalized to say "it" rather than he/she/they/them.
"Hey babes, look who it is!"
If we know the gender of the person first-hand then we might consider using: he/she/him/her/they/them.
"I think Bob is here. Go get him."
We can avoid the pronoun altogether and consider writing:
There's someone at the door. Can you go (and) see who's here?
"Hey Annie, look who is here!"