Because until you put it on record as he and Knight and Bass have done as well as Rosie and Ellen, many people believe you are straight. Denial/ignorance is huge in this country. Yes, we can all laugh and giggle and say, "Duh!!!!", but not everyone thinks that way or has gaydar or is in the know. Believe it or not, there are people who believed that everyone of those people I named were straight. And there are people who believe that others who have not come out officially are straight even though many of us already know (or think we know). And, it would be grand if people didn't have to come out--that they could just be, but that's not the reality of our world yet is it?
Coming out works in stages. For me, it was
--coming out to myself
--coming out to close friends
--coming out to family
--being out at work and school and in life
and one day if I were to ever be famous, despite all of you and the rest of my lil world knowing I'm gay, I'll have to officially come out "on the record" even though I haven't been in the closet for over a decade and will never be a closeted performer. That's just how it works. And I don't understand why this concept boggles so many people, especially gay people (and anyone with a secret) who know firsthand what coming out is like--the before and after.