You're right that in general it's about getting those random dopamine hits when something nice appears in the news feed.
However, after some time, you got a lot of the nice stuff and no exciting stuff appears anymore. At that point, you're still scrolling, hoping for a dopamine hit. It does not come because you are satiated, desensitized and the algorithm no longer has good stuff to offer you.
I get it here on Hacker News. After coming too often and scrolling too much, I already clicked on all the good links. All that is left is either not interesting, or stuff I've looked at before. I still scroll, doomed to find nothing. And yet I scroll.