I feel like the scroll fade fad is misunderstanding layered on bugs, turtles all the way down.

Once upon a time, developers implemented lazy loading of images, to save bandwidth. However, some developers implemented it poorly, waiting until the moment an image is scrolled on-screen to even start loading it, leading to a visible blip as you scroll.

(The better way would be to load an additional pageful of images beyond the current scroll view, which would provide enough time to load before scrolling into view at least most of the time. However, this doesn't maximally save bandwidth and some developers don't make good tradeoffs between diminishing returns on saving bandwidth vs. visibly degraded UX.)

Then, designers saw the blip-into-view effect, thought it was an intentional visual effect (rather than an artifact of poorly implemented lazy loading), but thought, oh, I'll fix it so it looks nice, with fading.

And here we are with a dumb visual fad originating from a bug without realizing it was a bug.