peeps on TV don't say hello or goodbye

Sorry to get here so late with the answer, but here it is:

Bad scripting. You don't have to say goodbye or hello when you already know not only who the other person is but also exactly when they are going to run out of lines.

Its also done in the interest of saving time--which is why characters on shows always understand each other, and nobody ever has to explain what they mean. Realism is sacrificed for dramatic effect.

The phenomenon is not restricted to dialogue. Ever notice on "Star Trek" the magical doors, which whoosh open whenever a cast member wants to leave the room, but stay shut if he or she is just walking nearby?

Greyfan's right. It's mainly a time-saver. I think I have heard TV and movie characters say "Hello" after picking up the phone, but hardly ever "Goodbye" or any other sign-off when hanging up. The point is, most dialogue isn't really life-like if you stop and analyze it. There are conventions of communication, strictly observed by the script writers to keep the plot moving along.