They do, but if you scroll past a post in your feed, and then try to scroll back to find it, there is very little guarantee that it will still be there.

When I worked at Meta there was a lot of employee pressure to implement a "deterministic feed", where scrolling back-and-forth would produce the same set of posts, but leadership constantly sandbagged the idea (nobody actually wants determinism, people prefer to be surprised/delighted, etc)