I think this generally applies to self-help style books, where the majority of the book is fluff with very little useful content. Those books can definitely be condensed into short blogs posts. However blogs and random tutorials don't do technical topics justice. If I'm learning how to reverse engineer a piece of malware or how to create a blockchain application, textbooks are the best resource to guide me.

To be fair a vast amount of technical books have the same problems, going over the same things again and again.

A couple of years ago, here o HN, an author shared his experience about collaborating with a publishing house. Apparently the publishers have very specific requirements about the required number of pages and the author has to conform.