The person finding an overlooked piece has brought a fresh eye to familiar confusion.

Long attention to a problem builds a picture, and the picture starts hiding things. Somebody arriving without that history sees the table rather than the story about the table, which is why they can pick up what everybody else has been looking past for an hour. It is not superior sight; it is the absence of accumulated assumptions.

When you are stuck, describe the problem out loud to someone with no stake in it and let them ask the obvious questions. The value is in what they have not yet learned to ignore.