I heard a podcast by a few fiction writers about writing magic systems in fantasy stories; the fantasy author was very big on having sensical rules for magic, and gave several reasons(Google 'writing excuses').
How does magic work in the world if it doesn't involve an energy source? I think the generic D&D assumption is that Arcane casters use a spell's VSM components to channel energy from Elsewhere into the Material Plane, with their will and the spell affecting the resulting release of energy. Item enchantment is a variant on this where that energy is slowly fed into a physical object, using arcane techniques related to various spells to align the energy within the item to cause various effects.
If a magic weapon contains no magic, how is it different from a non-magical weapon?
To go back to the sword analogy, 'forging' is a process that takes metal and produces a sword. You can't get the forging out of the sword because it's not a thing, it's a process; a shaping of the thing(bar of metal) into a weapon. In the same way, we assume 'enchantment' is a process that takes magical energy (xp in previous editions, Residuum in 4E) and shapes it into a magical effect within an item.
You could argue that 'enchantment' is a process that manipulates the object directly; but I don't see that creating magic items much better than a master craftsman; perhaps slightly better, but not much. And certainly no 'weird' effects, like flaming longswords, rods that shoot lightning bolts, etc. In this case, the 'magic' is that the blade is 'really sharp', or 'very well balanced'. Perhaps it's better than what a talented smith could do, but only a qualitative difference, not 'my sword bursts into flame to burn my enemies'. If you do allow for other effects, flaming swords, etc... then you get lots of 'where does the fire come from?' type questions, which, in my experience, never have as good answers as the 'the magic energy in the sword makes the flame' solution.