Interesting that you liked "The Sun Also Rises" but not "The Old Man and the Sea". Blows my theory away.
I always thought "The Old Man and the Sea" was perfect Hemingway. Poetry in the spare simple prose. A human in a futile fight, but not giving up, struggling every second of the way get what he needed from unforgiving nature to make up for what was not provided by an even more callous and unforgiving society. And the strength it took to fight that fight and the human suffering behind it. And while Hemingway usually portrayed manpain, to me "The Old Man and the Sea" was not about manpain, but about human pain in someone who happened to be a man. (In fairness to those who see this as manpain, the main character happened to be a man because Hemingway was incapable of portraying women as fully human except for very brief stretches.) To me, the "The Old Man and the Sea" is the one work where Hemingway slipped, probably accidentally, away from the portrayal of machismo to portraying the intense stoicism you sometimes find in the very poor who work hard in jobs where they undergo intense suffering. And the very short novel is not just about the sharks, but about the work of fishing, and about the old man, and about the old man's life.
I can understand not seeing this; the reader has as much to do with the tale as the teller. But if you were wondering what people who love that book get out of it, there is one data point for you.