Lots of books CAN be summarized in a few points.

Yes, that's true, but bullet point ideology isn't very useful imo.

For example you can easily sum up "How to Win Friends and Influence People" into a few short recommendations. "Empathize with people, listen closely to people, let them do most of the talking, be quick to be upfront and honest with your faults." But it gets tenuous and some of the exact meaning is lost, and the bullet points aren't very convincing on their own of the effectiveness of the recommended methods.

The book fleshes out the advice with clarifications as well as stories from Carnegie's life of times he used the techniques and their results.

I think humans are good at remembering stories. For example I always remember the story about him letting his dog off the leash in a dog park and how he leveraged his technique for getting off the hook when a cop confronted him about it. I think I remember that better than the single bullet point advice of "readily and quickly admit when you're wrong." In fact I think the end of each chapter succinctly states the exact tip and I'm sure I'm misstating it here.

I could use anki to memorize a summary but I don't think I'd be able to as effectively incorporate the techniques into my behavior without the working examples, stories, and evidence he provides.

And that's just non fiction. I can't fathom the point of summarizing a fiction book, whose entire enjoyment comes from the reading of it.