QUOTE(ihateregisteringeverywhere @ Nov 10 2008, 05:24 PM)

That's mostly because it's enough work creating custom items, most makers don't want to bother with the rest, which would also possibly conflict with other mods - lowering the value of the mod instead of increasing it.
You forgot to mention, that often those mods are released as resource mods, which ends up providing content for other modders to use. Having the ability to just download, merge, and utilize resources within a mod can make large projects, wanting a semi-unique look, MUCH easier to complete, especially since half of the time spent on these projects seems to be related primarily to creating those resources.
There is also the fact that many weapon and armor mods just don't make sense with the rest of the world, so even having one NPC walking around wearing those items could end up looking very wierd.
But yeah, it's mostly that it's easier, and faster to just place the items in a container, and stick them in a cell somewhere than it is to do anything else.