"All" really isn't true, most AM4 Ryzen chips don't have iGPU (unless they are specificly APUs), and a lot of Intel chips have popular 'F' SKUs without an iGPU.

The iGPU's video decoder could even be drastically less efficient than the dGPU's decoder, and it would still be better to use the iGPU, because just powering up the PCIe link and VRAM for the dGPU costs far more power than the video decoding itself.

What makes you think the unaccelerated path would use the accelerated iGPU path? Typically hardware decoding is disabled because of driver/reliability issues and thus would switch to software decoding to sidestep these concerns.

I assumed the OP/comment means "software vs. hardware accelerated rendering" when they say "it uses the CPU. And iGPU is still hardware accelerated rendering.

But some modern video codecs are designed to work quite okay with software rendering, especially if it's lower resolutions, and modern higher end dGPUs often have a pretty high "power overhead" on low utilization. To add to it the driver might keep it in a more high power state then strictly needed due to "expecting" more utilization, or similar.

Neither of the last two processors I've purchased in the last few years had any iGPU.

Not all, most desktop have two options with or without igpu.