probably so that if you see something you’ve already seen / liked, you know that you’ve been here before and don’t have to go further back in case you’ve missed something?

makes sense but why do the posts have to be in chronological order

because if they’re not, how do I know that I have seen all posts? Say I see all the posts as of right now, then come back one hour later. Now I want to read everything “since the last time”. Now I just read until I see I post I’ve already seen. This only works when chronologically ordered, right?

but what does it mean to “see all posts” if it’s an algorithmic feed? we can keep showing you relevant posts near-indefinitely. chronological only makes sense with a limited set of posts, such as “all posts from people you follow”. “any possibly-relevant post” is unlimited

yes, all posts from people I follow. That's why I follow them 😅