Why dark backgrounds in videos look awful

Even though typical video has 16.8 million colors (256 shades each of red, green, and blue. 256 * 3 = 16,777,216), large gradients of dark colors have a terrible blocky look on displays. Why is that? Tom Scott says there are three reasons: 1) there aren't a lot of dark colors, which makes it easy to see color bounding in gradients 2) the way human eyesight works, and 3) lossy compression.

Image: YouTube