Sometimes a fundamental idea is simple, but it takes repeated hammering into your mind before you adopt it. 95% of a book might seem like fluff, but when you read the 1 example, the 1 anecdote, or the 1 "way of saying it" that resonates with you - that's often when the idea sticks and becomes a more permanent part of your life.

Expanding an idea into book form helps more people discover that "aha" moment than a single blog post does. Plus, people just take books more seriously in general!