Same reason Netflix doesn't let you list all the movies on their site in alphabetical order - they want to create the sense that there is unlimited content. If every time you opened Instagram and there was no new content and you saw the same picture you saw last time you would just close the app. Now you never really know if there is no new content or not so you scroll down... and down...

It can’t be that simple, however, because all three show lots of old content for days even after you’ve looked at it. There must be some component of predicting the content you’re most likely to click on or guaranteeing views for promoted content.

It could also be that you only need a certain percent of new content to keep eyeballs. Thus, by only showing some new content at a time, instead of all at once (as in a chronological feed), you would increase user engagement time.

> I don't know why Snapchat, Instagram, Facebook, etc can't have a simple toggle between chronological and the default algorithmic feed...

It's not that they can't have it. They have no incentive to do so.

Algorithm feeds provides the guarantee that an ad is seen by a minimum number of users, which is the entire value of facebook.

What? You can put ads in chronological feeds too.

Fair enough. So, why doesn't facebook allow chronological ads?