What Does the Screen Do To Actors?

So since moving here, I have noticed something about celebrities.  Most of them are far better looking in person than on screen!  It is actually something that is really interesting to me.  I don't understand what one looses between going from their trailer, to makeup/hair, to the screen.  It's is baffling, really.  

Today I saw Orlando Bloom.  Never really thought much of the guy (aside from his hilarious -yet unattractive- appearance on Ricky Gervais's show 'Extras').  But today I was running my usual 5-6pm run on Runyon Canyon, and there he was - stunningly attractive!  It was another moment where I thought, "oh, THAT'S why he's famous."

But Orlando isn't the only famous person who is subjected to this...  Ryan Seacrest, Kimora Lee Simmons, Lance Bass.... the list could go on.  (AND TRUST ME... there are also the people who look worse in person, but I'll save the space)