One claim I've seen is that it is because they are frontlit rather than backlit. But to the photosensor cells that sense the light and report to the parts of the brain that control circadian rhythms, it should just be frequency and intensity that matters. Light of a given frequency and intensity should have the same effect regardless of whether it came from a frontlit or a backlit display.

I have noticed that the brightness I need to comfortably read a normal screen seems to be higher than the brightness I need to comfortably read paper. Perhaps that is the difference--we simply use less light with paper?

Or perhaps with paper, where we get to choose what light to use for our nighttime reading, and it is often done with a light dedicated specifically to that task, we tend to choose low blue lights?