Since when? :haha:
You may find an old barrel that somebody unbreeched, and then simply screwed back in..., and they might unscrew with a wrench in one hand and the barrel in the other, and yes you can return that to the same tightness. But my experience with antique guns that are properly breeched is that you need a barrel vise and a wrench to get them out, and to re-align them back into original position. I wouldn't assume that since the gun was intact, and the breech plug can be removed and replace by hand, that such is how the gun should be.
Now I haven't had to unbreech more than a dozen barrels in my time, so I suppose I got ten or so that weren't the norm...,
As far as why modern folks don't do this..., it's liability in some cases, and in others, because of the gun style, they wouldn't want you removing the barrel every time. Not all barrels use keys and a hook breech, or even a tang screw. Some guns are SxS, caplock or flintlock, and they especially would not want the shooter to unbreech the barrels.
LD