Thomas writes a letter to his niece, Mary, stating his intention to give her his vintage car as a graduation gift. The letter includes a request for Mary to sign an enclosed copy of the letter acknowledging receipt of the gift. Thomas keeps the car in his garage and retains the keys, but he hands the signed letter and the car's original title document to Mary during a family dinner. Mary, unaware of the car's value and not fully attentive, casually places the letter and title document in her bag without acknowledging the gift verbally or in writing. A week later, Thomas passes away. Mary finds the letter and title document while sorting through her belongings and claims the car as her gift. Thomas's will, however, does not mention the car or any gift to Mary.
If Mary is unsuccessful at arguing that Thomas gave her the vintage car as a gift, what is the most likely reason why?