My general point is that books/longreads require a lot more research/thought to produce, which takes more time, which in turn pushes you into making that one piece of writing the best it could possibly be. This is somewhat lost in the newsletter model, for reasons I don't fully understand yet. One big one is that people get newsletters into their emails, and they may not be on average ready to dive deep into a topic you want them to dive into. So perhaps newsletters just send stuff that has the highest email open rate- which probably happens to be lightweight daily updates.

This is different from a book or even Medium, where people come already mentally prepared to read something deeper and more insightful.