Because the Vatican says it doesn’t. Or something.
No one really believes that two men can marry, and this includes those homosexuals deemed “married” in states that entertain this fiction. But a separate matter is the right of homosexuals, and their heterosexual supporters, to declare that Catholicism is somehow agnostic on this issue. It is not. There is only one teaching body in the Catholic Church, the Magisterium, and it neither recognizes nor sanctions social arrangements that compete with marriage.
You know what really doesn’t exist?