This is a great question. You might find you get a range of answers, but my own idea is that we see in this section of The Odyssey an Odysseus who displays his customary flair for storytelling. It is as if he loves to weave stories for other people. It is important to note that although, strictly, the story that he tells Eumaeus is a "lie," actually, Odysseus incorporates many actual adventures into this "lie," making it a strange commingling of truth and fiction. Of couse, we must remember too, that Odysseus is returning to his kingdom after a massive length of time away from his wife and his people, and so we can understand why he is very wary about being honest and revealing himself, even to the most loyal of his subjects such as Eumaeus. You always get the sense, though, that Odysseus loves telling stories so much that he gets somewhat carried away. He could have revealed himself, as you say, but delights in testing, and telling stories, so much, that he delays.