...Because it's loaded, and that's dangerous...
But that's exactly the problem I see with the mantra: the danger is linked so directly to only that one thing - the (real or imagined) loaded status.
Not the pointing in an unsafe direction,
Not the finger on the trigger,
Not the target and backstop knowledge,
...you know, the actual rules that should be followed regardless.
I'll never see how, on a rational basis, this is somehow superior to drilling those 3 rules regardless of assumed status.
But whatever works for others.