Not sure I agree with you. An Instagram/Facebook/etc. feed is not like a book or movie that has one continuous narrative at the backbone of every smaller story being displayed.
In this sense, social media is more about mimicking real life occurrences than telling a complete narrative.
Just like a Facebook feed, our brain perceives certain daily occurrences with grater focus and interest than others. It would be wrong to provide equal weight to all of them based on a chronological order. I wouldn’t want to reflect on a boring bus ride I had.
I do agree that it is annoying that certain posts disappear when you try to return to them. Currently, you can save a post but I guess what is required is a better algorithm that keeps the posts it thinks you would like to return to or a better UX that allows you to return to your feed as it was.