Impressions are marbles

I don't get why impressions of famous people are funny. Impressive yes, but funny no. I got to thinking of this a few weeks ago when I saw the guy from Frank TV at a random campus bar on a random Saturday afternoon. He was a cool guy and let us take a picture (dude on the right) and even did a quick W impression. He does one hell of an impression, and I laughed, but I don't know why.
I have the same problem with SNL. Their better known sketches over the years seem to have centered around direct imitations of famous people, doing what they normally do, the same way they normally do it. Sure, there's some exaggeration, and that is of course necessary in order to "make fun," but it seems like a lot of audience's appreciation gets bound up in how spot-on the impression is. And I just don't see what it is about direct imitation that is funny (even though I laugh more often than not).

Take the Tina Fey-Sarah Palin thing. First, it was clear from the beginning that the bitch just looked like Tina Fey (and Tina Fey gets first dibs on looking like Tina Fey). And when Tina Fey did her now-famous impression, the likeness was uncanny. But it was seen as especially remarkable because she was able to quote the would-be-VP-disaster verbatim and still get huge laughs. That probably says more about her subject than anything else, but why don't we all just laugh directly at Sarah Palin? That shit is funny out of anybody's mouth, if you ask me.