Because our energy consumption is ultimately tied to the exponential growth of the economy. Even if everything around us would become 10x more efficient - which already would require many scientific breakthroughs - that's just multiplying the exponential growth curve by a constant factor of 0.1. A little bit of relief, but it fundamentally doesn't change anything.

And that's ignoring the reality that if you make something 10x efficient, it tends to become used more - sometimes much more, cancelling the gains. Our economy is self-regulating; altering one factor only makes all others readjust to compensate. Permanent change will require more complex approach.

Energy efficiency is definitely a very important part of dealing with climate change, but it isn't a silver bullet.