A postcard filled with cleverly crafted words can alone help you master business strategy, competitive analysis, economics, corporate laws, finance, marketing, sales, and more. Or can it? A paragraph is no substitute for experiencing the world, yet, a summary promises precisely that.
Summaries present two problems.
First, summaries tend to confirm biases. If I agree with a book's synopsis, I might assume that the book's 150-page argument is sound. The quantity of words, rather than the quality of the argument, would drive the validity of my belief.
Second, summaries blur the line between the concise and the shallow. Knowing that "engineering is the art of building things" is no substitute for going out and creating stuff. Similarly, a summary does not make a book concise but shallow.
In short, there's no substitute for you experiencing the world.