It all comes down to the definition of a tag. Emphasis mine. Explaining why the rules are written that way is just speculation.
TAG is the action of a fielder in touching a base with the body while holding the ball securely and firmly in the hand or glove; or touching a runner with the ball or with the hand or glove holding the ball, while holding the ball securely and firmly in the hand or glove. It is not a tag, however, if simultaneously or immediately following his touching a base or touching a runner, the fielder drops the ball. In establishing the validity of the tag, the fielder shall hold the ball long enough to prove that he has complete control of the ball. If the fielder has made a tag and drops the ball while in the act of making a throw following the tag, the tag shall be adjudged to have been made. For purposes of this definition any jewelry being worn by a player (e.g., necklaces, bracelets, etc.) shall not constitute a part of the player’s body.