Blake Shelton admits he's not much of an actor.  In his videos, Blake usually performs the song and lets actors tell the story.  But, his lack of acting chops isn't the only reason for letting the professionals handle the acting.  Shelton does not want to see a random actress portray his love interest.  He tells People:

“Everybody knows I’m married to Miranda Lambert, and they don’t want to see me leaving a bar with a girl. Why would I take this ring off for a video? I’d be insulted by that if I was Miranda, and I don’t want to do anything to insult her or hurt her feelings.”

Shelton does say that he'd do the acting if Lambert was in the video.