Why give each Person a signature Action and Object?

Because their natural, iconic move and prop are far easier to recall than arbitrary ones.

If the Person is a tennis star, their Action "serving" and Object "racket" almost pick themselves — and stick, because they belong together. Choosing each item's Action and Object to match its Person makes the base images feel natural and memorable. (In a composite you'll mix slots across items, but a strong, characteristic base action/object for each item is what makes those mixes vivid and unambiguous.)