> Does anyone know why Linux laptop battery life is so bad?

It's extremely dependent on the hardware and driver quality. On ARM and contemporary x86 that's even more true, because (among other things) laptops suspend individual devices ("suspend-to-idle" or "S0ix" or "Modern Standby"), and any one device failing to suspend properly has a disproportionate impact.

That said, to a first approximation, this is a case where different people have wildly different experiences, and people who buy high-end well-supported hardware experience a completely different world than people who install Linux on whatever random hardware they have. For instance, Linux on a ThinkPad has excellent battery life, sometimes exceeding Windows.

Are there any repositories of documented battery life behavior?