The book effectively says that people struggle with these things like lack of purpose because of things your parents didn't do (and probably didn't know to do). So, you know something is missing, but don't know what since it never happened.
The central premise of the book is that emotions are fuel for your life and if your parents didn't teach you how to understand and manage your emotions (or taught you they weren't important), then you will likely feel emotionally empty or emotionally numb. Since emotions are what give color to the world, you will struggle to manage your reward systems in ways that "fill you up."
I feel like "why is it hard for people to know what they want?" is a re-wording of "how do I find my purpose?" which is a rephrasing of "I feel empty," which is a rephrasing of "I am emotionally numb."
You might find relevant responses here: https://news.ycombinator.com/item?id=33534644
It is worth reading running on empty to see if it resonates with you. I found it satisfying, mechanical, direct, and most importantly devoid of vagueness or platitudes. Very good book.