It's really a testament to just how large the ocean is. Remember there's more than twice as much sea than land and we lose things on land too. Things on land might not be under 3 miles of water that takes expensive, specialized equipment in remote locations to actually reach.

I do hope for a resolution for MH370 one day but we know the bullet points: it was a deliberate action by the pilot (and/or the co-pilot) to depressurize the plane, run it out into the ocean and (most likely) make a "soft" landing after which it would fill with water and sink. There's alot of evidence for this and no other proposed scenario fits the evidence.