Why is the Pandemic starting position always Atlanta?
Because Atlanta is the home of the CDC. And since we all live in America they are responsible for diseases everywhere...

EDIT: I just checked the English and German rules and now understand your confusion. The German rules explicitly say that this is why players start in Atlanta. For some reason the English rules skip that part.


  Danach nimmt sich jeder Spieler die farblich passende Spielfigur und stellt sie in Atlanta auf, wo sich das Zentrum für Seuchenkontrolle und -prävention (Centers for Disease Control and Prevention) befindet.


Which roughly translates to:


  Then every player takes the matching pawn and places it on Atlanta - where the CDC is based.