Because that would just make them another boring straight couple. To paraphrase Paul Mooney "Everyone wants to be queer, but nobody wants to be homosexual". These people are happy to adopt the label to make themselves interesting, knowing full well they'll never have to face any of the actual implications of being gay. They'll never have to not hold hands in certain neighbourhoods, they'll never be at risk of a gay bashing. Because to all outside observers they are very obviously a straight couple.