This exact dialogue takes place in real life regularly.

One reason that it is funny is that it plays against that.

We the audience maybe forget for a moment that we are not watching real life. We are watching a drama or entertainment. So we expect something relevant to happen. That’s the convention.

The exchange plays with that expectation. It deliberately forces us out of our pleasant illusion and makes think us about our real experience - we are sitting in a seat and watching a performance, which is happening at that moment.

And nothing happens, just the same as real life