I didn't see him / I haven't seen him

Question

Watching a movie, I've just heard of this lines:

A: "I'm just looking for Barbie. Is he around?"

B: "I haven't seen him."

I've always been confused about present perfect and simple past. So why did B say so, not "I didn't see him." What's the difference?

^{The dialogue appears to be from the first episode of Under the Dome and "Barbie" is the nickname for a male character named Dale 'Barbie' Barbara.}

Answer 1

I haven't seen him

means, essentially, I have not seen him recently.

I didn't see him

means, I did not see him at some particular time.

When the "particular time" is recently, the two can be used almost interchangeably:

"Have you seen Bob around?"
"No, I haven't seen him."

"Did you see Bob just walk by here?"
"No, I didn't see him."

Other times, though, there are clearer dividing lines between when to use didn't see instead of haven't seen:

"Have you seen Mr. Hernandez go into Mrs. Olson's house?"
"Yes, I have seen that many times."

"On the night of the murder, did you see Mr. Hernandez go into Mrs. Olson's house?"
"Yes, I did see that."

The first question refers to a more open-ended span of time, while the second refers to a very specific time segment.

Answer 2

In this context, did is past and this means that some time in past, the answerer has not seen Barbie.

Using have extends the time from past into the present. I haven't seen... here means till the time the sentence is spoken, the answerer has not seen Barbie.