Flat histograms indicate that each tone in the range that the image could contain occurs equally often as any other tone.
A continuous wedge would give such a tone frequency distribution.
Or a scene in which the tones are distributed in another pattern (preferably one that represents a thing in real life), yet by some magic all appear as often as any other tone. Could happen though. But repeatedly, in different images of different scenes???
Add in colour and consider that to have a flat line overall histogram, the histograms of all colour channels involved would also need to be flat, and it becomes even more difficult to imagine.