They used to do that. They had two different news feeds, one with their algorithm and one that was just a real-time feed. Then they scrapped the real-time feed in favor of their algorithm-based feed. It's not that the real-time was hard to do, it's that the algorithm is more easily monetized.
Moving the feed from 'chronological' to 'algorithmic' obfuscates the true quantity of actual content on the user's timeline, thereby allowing a higher density of ads and other sponsored content without the user necessarily being able to tell or prove.
I hypothesized about other benefits and drawbacks here [1].
Yup. It was a huge deal in, what, 2008, when they did this? Almost as big as when they got rid of curated content and turned everyone's link-able "interests" into weird generic "likes" (it was a really strange period for at least a few months as I recall).
For awhile you could switch back every time you logged in, manually, to a chronological news feed.
Instagram was one of the last holdouts but they switched a year or two ago.
Twitter is amazingly bad at this. It takes screenfuls of junk on mobile to get to the chronological stuff.
