"A shotgun...... with one in the chamber is generally as safe as a wheelgun."
Generally not true, with the possible exception of Mossbergs. Remington pattern guns and similar do not have a firing pin lock or any mechanism to lock the hammer and/or sear. They are not drop safe and may discharge if the is a round in the chamber and the gun is dropped or struck.
"Oh for God's sake. When you pick up the gun, you rack it"
Again, generally not true. The chamber is charged when the gun needs to be fired, not when it is picked up. There is plenty of time to rack the action while mounting the gun to a firing position.
Most of us will lay hands on the gun far, far more often than we will need to fire it. Prematurely chambering a round just provides more opportunity to a drop- or strike-induced unintentional discharge