I would assume the main reason is one of game balance. Especially now with the
eqsets, 'regen' items would otherwise never need repairing. But if you want a
more RPG reason, let's see if any of these work:
-- the monster tripped you and you fell down hard
-- you were hit by an area-effect blast of some kind
-- the monster's attacking limb is so big it can hit your entire body at once.
For example, a giant's mace or an acid blob's enveloping attack.
-- your magical weapon, when damaged, released some of its power in a blast
that hit all your other gear
-- you crit the monster so hard that shrapnel flies off and hits you
-- you were struck by a magical spell, which causes a small explosion
-- those racial gods you haven't been donating to have decided to step in and
show you the danger of not praying to them

All that said, I will stick with my original answer, that I think the "hit all
items" strikes are largely intended for game balance, to make combat sense and
merchants more valuable to the game.

Shinarae Lluminus