Why is the object of knowledge "what is real"?

Plato's quest for knowledge originates in his conception of the debate between his master, Socrates, and the Sophists who were selling their wares in the Athenian marketplace. What Socrates sought was genuine knowledge or wisdom, and what the Sophists sold for a fee in the marketplace was an "illusion," not the real thing, a phony counterfeit that would deceive the ignorant.

Thus epistemology begins with the contrast between genuine knowledge versus mere opinion.  Knowledge is distinct from opinion in that "knowledge" is of what is really so, while fallible "belief" is of what appears to be so.

From this contrast Plato concludes that when one has knowledge, what it is that is known, what is called in epistemology "the object" of knowledge, must be real.