Several reasons occur, but they all fall into conspiracy territory.
Best one: it’s in a hangar, refitted with W8 nukes, ready for ISIS to fly into Vatican, Moscow, Beijing etc.
Hard to imagine more debris wasn’t found, unless it landed/crashed on land, and ocean search is a goose chase.
The ocean eats things. Look how often we find lost boats from over a century ago. And those are in coastal waters. Things that sink into the middle of nowhere are GONE. Most of the debris will sink, and if the plane is ever seen again it will be 200 years from now because somebody is drilling for something in just the right place. If it was “stolen” (which would be a pointless waste of effort since there are literally thousands of planes in long term storage available for purchase by basically anybody) they’d have used it by now.