Even while not impaired nor distracted, I have a much easier time seeing a decently-lit cyclist than I do seeing a reflector.

It's the same way with parked cars -- a car with its lights on is just plain easier to see than one with them turned off.

Much of a car's headlight power is restricted, too. Their low beams are required to not shine directly into oncoming traffic, so they have a cutoff pattern that essentially blocks almost all of their output from projecting any higher than the headlights themselves. The automotive headlight output that actually hits a higher-mounted bicycle reflector, then, is fairly minimal -- and less than an actual bicycle light.