Put it this way. If you're trying to push something by hand, and you want to keep pushing it, you can just put more effort into it.
If you're trying to push something with a motorized vehicle, and that vehicle runs out of power, you can't just keep the vehicle moving with your own effort.
The power in an enchanted item comes from the item, not the caster, and an enchanted item isn't made to focus the caster's power through it--and that's what such a duration enhancement would be, trying to cast through the item.