I admit that I’ve often wondered this myself. Rod Dreher ponders:

I genuinely don’t understand [Andrew Sullivan’s] position. He doesn’t believe the Catholic Church teaches truth, except insofar as it coincides with what he believes. Staying inside the Catholic Church makes him truly miserable. So why stay? If he wants liturgy, smells, bells, and a complete blessing on the way he chooses to live his life, there’s the Episcopal Church. I actually did believe in Catholicism, but for my own reasons was so tormented by staying that I lost my faith … and so I left. I left in tears and heartbreak, but I left. Truly, it’s a mystery to me why any free man would stay in a church in which he did not believe, and that made him so unhappy.

Here’s Sully’s unconvincing riposte:

I can recite the Creed with as clear a conscience as any of my fellow Catholics. I do believe that the Catholic church teaches truth in the single unifying credo I can recite at every mass. I do believe in the message of the Gospels as deeply as I believe in anything. I do believe in the Catholic communion as the core guardian of those Gospels and of the sacraments that keep Jesus in our tangible, physical midst. And I do believe in the task of spreading God’s love as the core mission of a Catholic today. What I do not believe in are the Church’s contemporary social and reactionary political positions, and its cultural hostility to women and gays, and its profound ethical corruption and sexual hypocrisy, all of which have led to astonishing scandal and evil. I do not believe that this evil should be tolerated or enabled by those who love it. And I do not believe that tackling this evil is best accomplished by leaving, as Rod, for reasons that I deeply respect, has.

For now, I won’t delve too deeply into what I view as the various defects in our evolution as a species that lead people to seek out and hold onto religion, but I will posit that I’ve always seen Sullivan as hanging onto his Catholicism for two reasons:

like many people, he has a sentimentalist attachment to something that was powerful in his life as a child; and his deeply contrarian nature binds him tightly to the things within himself that the outside world finds to be naturally incompatible.

Just my two cents.