Is this true or an illusion? If true, why?
Well I don't think the amount of gay people involved in the arts is as high as you seem to think. However, I would tend to agree that homosexuals find more success in the arts. The main reason is that artists by nature are contrary to the rest of society, so a flamboyantly gay man like Liberace or Andy Warhol (I like rocking it 60s style) is more easily embraced by the artistic community so those artists don't need to worry about keeping up appearances, which allows them to concentrate on their craft. As far as actors go, the Rock Hudsons and Robert Reeds had success because as far as they were concerned, their public faces were just another character so it was a chance for them to practice their craft, and you did have exceptions to the rule like Paul Lynde and Jim Neighbors who were so gay they made an Easter basket look boring in comparison, but they never carried roles of significant weight or importance, so middle America just looked at them as odd curiosities that was freeing in its own right for an artist.