Three reasons:
Out-of-universe, it's too easy a fix; the White Court just aren't able to live as "normal people".
In-universe, if this was possible, why didn't Harry build something like this for Thomas when he was fighting his Hunger between BR and TC? Even if Harry didn't know enough about the Hunger's nature, Lara does; why didn't she suggest Harry do thaumaturgy to feed raw magic to Thomas' Hunger in PT?
In-universe, it doesn't really fit the way the feeding is described to work IMO. It's been described as merging auras with the prey. I think it needs an actual coherent aura/spirit of a sentient living being*, not just "X quantity of life-magic energy".
*Maybe a close-enough-to-mortal being ... we've seen it used on part-humans in "Backup" and "Bigfoot on Campus", but never on totally-inhuman things. Kind of doubt it would work on Bob or Kalshazzak or an Outsider.
Not quite. I think what is being consumed is "generic" life force energy - not specifically fear energy or lust energy. But the mode of access to that energy isn't compatible with anything other than a near-human being... an item can't work since the item itself doesn't have a spirit or experience emotion.
Basically, yeah... Thomas' Hunger demon can eat his own life force directly. So the emotions provide access to energy not energy itself.
It's not like a person has separate stores of "lust energy" and "fear energy" and "despair energy" and so on. A person has a certain amount of life force, and there are different ways to access it. (Wizards & other spellcasters can draw on their own life force, sometimes using emotions, sometimes not.)
Someone drained to zero by the White Court dies; they don't become dispassionate or fearless or whatever. And in "Bigfoot on Campus" we see that a being with more life energy than the Hunger Demon can handle can survive a normally-fatal feeding. That pretty much confirms that it's "life force" not specifically "emotion type" energy.
I still think you're misunderstanding the scope of this project. It's not to replace feeding. It's not to make feeding obsolete. In fact, it's still dependent on feeding normally, to stock up the item.
It's more like an energy bar for the demon to munch on if things get desperate. It could be used to continue a fight (briefly) that otherwise would be lost due to running out of energy, or to prevent the demon from turning on the host until they can get a regular feeding.
It wouldn't do anything to change the lifestyle of the wamp. It's not an alternative, it's a backup.
As for the energy, we'll just have to disagree on the storagability of the energy the wamp draws and feeds from.
The wamp would still be drawing out the energy in the usual method themselves. A little bit would just be directed to the item, which would store it as it is. Then, if they tapped into it, the energy would come out just like it went in. Just like other energy storage we've seen.