Why is realism incompatible with abstraction in epistemology?

Why does Objectivism assert that realism (or "intrinsicism") is incompatible with concept formation or abstraction? In "Introduction to Objectivist Epistemology," Ayn Rand says that realism (whether moderate or extreme) regards "the referents of concepts as intrinsic, i.e., as 'universals' inherent in things (either as archetypes or as metaphysical essences), as special existents unrelated to man's consciousness—to be perceived by man directly, like any other kind of concrete existents, but perceived by some non-sensory or extra-sensory means." Why couldn't essences (as posited by realism) be grasped via abstraction? Why must the grasp thereof be direct? Also, why is the realist view of concepts referred to by Objectivists as "intrinsicism"?