Thinking a bit more about this...
It's also visual fatigue, I think. How many photos in the same genre can you view in a row before you start to lose your ability to discern differences? (Similar to how many perfumes you can sniff before you can't tell the differences.) Before the internet age, we never saw so many images all at once (and TV was all low-res and moving images). The most we could see in a "stream" would be a coffee table photo book, and even that, I doubt most people would look through all at once. Now we get more new images than that each time we log into Instragram, even if we log in several times a day. Our brains are struggling to catch up.
(Yeah, yeah, people born and raised in the internet era are likely to be more comfortable with scanning large streams of images, but "comfort" and "scanning" are not what I'm talking about. They make you neither better nor worse at grokking individual images.)