It's pretty much impossible for Riot to make that work.
First: What other stats besides energy would you put on such an item? Shen's a tank, Zed's an AD assassin, Akali's an AP assassin, Kenen is a mage, and Lee Sin is an AD fighter.
Lee Sin and Zed are the only 2 that really consider similar items in their core build.
Second: the most limiting items in the game (that aren't champion-specific) are
, which offer something that's completly useless to about 50% of the champions in-game.
Mana items offer stats that around 80% of the champions in LoL can utilize.
An energy-exclusive item would only be usable by ~4% of the champions in LoL.
Third: a good item on a character and a usable item on a character are completely different things. While all champions have physical auto-attacks and can benefit from Last Whisper's AD and %armor penetration, only ~30% of the champions in LoL really feel that item is optimal enough to go for it.
Or for a more extreme example - only ~5 champions in the game actually want to use
, despite a solid ~60 being able to put all it's bonuses to use.
Apply all that to an energy item - and you're realistically looking at a scenario where only 1 or 2 of the ninjas feels that item is worth grabbing.
Which lends itself more to cluttering the shop and adding unnecessary burden of knowledge issues to the game, what with well over a 90% chance the item is unusable on any given champion.
Lastly - energy isn't the only alternate resource method, so adding an item that benefits energy regen champions sends an awkward message in regards to those alternate resource champions (health, flow, fury, ferocity, heat).
Finally: there are energy runes in the shop, and Blue Buff offers a considerable boost to energy regeneration - if you're having energy regen woes just try those out.
Or work on proper resource management, which most other champions have more extreme variants of with their respective ability costs.
If you don't feel like grabbing energy regen runes because missing out on what other runes offer, you're not making a good case for an energy item existing.