Because it depends on the show you are starring in. If the show happens to want to use your real name then fine, but most tv shows change the person's name.
Gianna
Marie
THE SCENE
@selenagomez
uhm .. Selena Gomez .?
Selena Marie Gomez
Monte Carlo
Her characters name is Faith.
Amanda Cornet
Her nick names are Sel or Selly.
Gianna is the name of Selena Gomez's character that she plays on Barney and Friends. :)
Her full name is Carter Mason.