Are there no actresses in the U.S?

A friend had me watching an episode of this cbs series, called ncis-la; I'm not much of a fan of it...but it kinda surprised me that they'd cast this mexican droopy-eyed girl to play that which they seemed to show as a white girl part, ( the name's kensi blygh, I suppose that's a white character's name)- and she's not even like a convincing actress...aren't there any white american actresses who can act in the U.S anymore? Why would someone cast an actress who pulls the whole show down?