User blog comment:The Milkman/Indoctrination Theorists Say the Darndest Things, Part II/@comment-98.240.62.123-20121205130042/@comment-24174486-20121205185555

Why is Destroy the only ending in which Shepard can appear, breathing?

Because Destroy is the only ending where Shepard has a body in which to breathe through.

It's really as simple as that. Control and Synthesis invariably require the destruction of Shepard's body.