You missed basically all of the practical details of sending and receiving mail. The people who wrote these RFCs and mail systems have been doing this for decad

You missed basically all of the practical details of sending and receiving mail. The people who wrote these RFCs and mail systems have been doing this for decades and have the experience to avoid the "why don't we simply [[wave arms]]" traps. The reason that DKIM signs only enumerated headers is because mailers add more headers as the message is in transit, and there is no way to sign those.