Why are furnace/central AC blowers so inefficient?
Okay, so I think I figured out the reason, and I learned a lot about HVAC in the process.

The answer is that a ceiling fan is moving air at basically zero "static pressure." Static pressure in an HVAC context means the amount of resistance that the air has to moving. In a free environment, that's zero, or close to it, but in a tightly restricted system of ductwork, it will be much higher. A ceiling fan is rated at such-and-such CFMs... but at zero static pressure. They don't tell you this because it's obvious; you're probably not mounting your ceiling fan in the opening of a four-foot-diameter cylindrical duct. If you found a curve of CFM versus static pressure for any given fan, it would probably drop off steeply. But of course there would be no reason to because the whole point of a ceiling fan is not to push  air through a duct, but to circulate it in an open room, so the motor is optimized for that use case.

This is compounded by the fact that most ductwork is terribly done, so furnace manufacturers err on the side of oversizing the blower to make sure the unit isn't starved of air (causing it to operate poorly or even die early), or incapable of supplying the living environment with the required heat. Either of these things would make people blame the furnace and make the furnace company look bad.

So the short answer is that you have to use a powerful blower in an HVAC system because ducts suck. A ductless unit/system could probably get away with a much less powerful blower because it wouldn't have to work nearly so hard.