The word "celebrities" is in quotes to denote the fact that we dream of celebrities for their symbolic value as a representation of someone else in the same mold.
"Bill Cosby" = famous comedian = Richard Jeni
"Don Knotts" = 1960's comedic actor = Marty Engels, 1960's comedic actor
I've never accurately predicted the death of a celebrity because that would be nosey and they are protected behind the shadow of a "symbolic representation" of themselves. There are other various reasons why we see celebrities in our dreams.