It's absolutely not the case that nobody was thinking about computer power use. The Energy Star program had been around for around 15 years at that point and even had an EU-US agreement, and that was sitting alongside the EU's own energy program. Getting an 80Plus-certified power supply was already common advice to anyone custom-building a PC which was by far the primary group of users doing Bitcoin mining before it had any kind of mainstream attention. And the original Bitcoin PDF includes the phrase "In our case, it is CPU time and electricity that is expended.", despite not going in-depth (it doesn't go in-depth on anything).
The late 00s weren't the late 90s where the most common OS in use did not support CPU idle without third party tooling hacking it in.