> The market of buyers will automatically reward sellers who develop more efficient devices once it starts hitting their wallet.
The point is that it's not "hitting their wallet." Idle power draw is an externality — it's negligible per individual (i.e. something people neglect to care about), yet adds up to real money at the municipal level.
And yes, you can make electricity cost more until it's not nelgigible. But you (as a tax-code legislator) don't want to do that. With economic "force-multiplier" utilities like electricity or gasoline, you don't want consumption to stay constant — you actually want it to increase, because consumption of such utilities is in the direct causal chain for GDP growth. Spending energy is necessary-but-not-sufficient to drive your country's economy; disincentivizing your population as a whole from spending energy, is disincentivizing them from driving the economy!
An electricity tax is like a corporate income tax: it indirectly disincentivizes people from making money. Which, as a tax-code legislator, is the last thing you want.
The point of this weird and arcane approach to incentivization that you object to, is to simultaneously incentivize useful electricity consumption, while also disincentivizing useless electricity consumption. To ramp up productivity, by saying "use as much of this as you can — but only in such a way as to drive the economy!"