You're right in principle, but in practice even factoring in externalities electricity prices won't be high enough for people to care. Using current US carbon intensity for electricity generation[1] and the higher end estimates for the social cost of carbon[2] gets us carbon costs of $0.142 per kWh. The average prices in US is $0.168. Adding in carbon costs would almost double the price, but there are countries with even higher electricity prices[4] and they're not exactly switching to more efficient software in droves to save energy.
[1] https://emissionsindex.org/
[2] https://en.wikipedia.org/wiki/Social_cost_of_carbon#Carbon_p...
[3] https://www.bls.gov/regions/midwest/data/averageenergyprices...
[4] https://www.statista.com/statistics/263492/electricity-price...
The reason it is not “invoked” is because energy prices are sufficiently low (due to not pricing in externalities) that there exists little incentive for end users to optimize for power usage.