Can you elaborate why these architectures give better battery life?

I don't think I know enough to give a great answer.

The obvious is TSMC's "7nm" and iterative 6nm enables greater efficiency over "Intel 7." Beyond that, presumably there's some IPC advantage that means more work can be done with the same clocks and power draw.

It's odd, because 12th gen is quite performant and seems efficient, and yet somehow the battery life isn't very good.

Intel's 1240P and AMD's 6600U perform more or less the same at the same power.

I'm not sure about the AMD option, but I can speak to the Intel trade-offs. H-Series CPUs are designed to be the "laptop workhorse" at a TDP of about 47W - whereas most laptops built for battery life nowadays use U-Series CPUs with a TDP of 15-17W depending on the generation.