Specifically, modern formats use gradients where possible. If something transitions smoothly from one color to another, they can represent that as what it is -

Specifically, modern formats use gradients where possible. If something transitions smoothly from one color to another, they can represent that as what it is - to oversimplify, one pixel is the first color, a different pixel some distance away is the second color, and the decompression will generate the intermediate colors for all the pixels between those two. By manually dithering, it has to encode each pixel individually, because the transitions are gone.