Why? Because all of the damn stats are on items.
Passives, actives, and who can get LS easily on a weapon without compromising serious damage stats come into play also though. (Skorn)
You make a good point but it's not as black and white as 30% reduction or not.
Using Tizani's example, if you make a TR/Bell monk without LS on Skorn it's going to suck a lot.
So the DH going to the monk gear is effectively freeing the Gloom slot for another attack and only giving up a LS slot on weapon in trade.