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There's plenty of reasons. They can range from things like Depression, Anxiety, ADHD or Executive Function Disorder.

It can also just be a matter of not encountering tools or techniques that they can apply to their daily life. As great as a book or system can be, it doesn't really do any good if you can't figure out how to apply it to your own life.

Saying that people lack discipline can even be counter productive. It is incredibly exhausting to hear that you're undisciplined while you're simultaneously fighting to accomplish a fraction of things you want to do, much less need to do. Once you've heard this enough, it can seem futile to even try, and you can start to wonder if there's something wrong with you.

I've personally experienced this while dealing with ADHD and Depression in my life, but, as my grandfather used to say "Different strokes for different folks"