We know you're bicurious.
Perhaps the reason bi celebrities may be reluctant to come out is that they fear it would hurt their careers in a way that being gay no longer does. Perhaps they fear their fans will wonder whether they are confused, promiscuous, or indecisive. That's what people wonder about non-celebrities, after all.
When I came out as bisexual, I heard responses like, "It's just a stepping stone between identifying as straight and identifying as gay," and, "See how you identify in five years." I learned that if a person does not fit in the straight-gay binary, some people won't believe them.