Sharp, contrasting edges are not common in nature, and they do not fair well when JPEG compression is pushed to its limits. Subjects consisting of black lines on a white background degrade rapidly. Similar things such as typefaces and line art are poor subjects for JPEG compression as well. Water, sky and skin can be generously compressed with the minimum of loss and retain their rich, true colors. We humans perceive the images around us in a reasonably well understood way.
Science understands the mechanisms of sight and the psychophysics of visual perception.
Our brains perform all kinds of tricks to improve our perception of the world around us. For instance, we perceive movies and television like the real thing, even though the movie screen displays a short burst of a still picture followed by a relative eternity of total darkness before the next burst. Our TVs have red, green and blue dots, but we see lots of different colors. I have a retinal condition which is only apparent when reading print, yet I perceive no blind spots when viewing natural scenes, because my brain can predict the missing material and fill in the gaps for me. We perceive small changes in brightness more readily than we do small changes in color.