About 2.8% of the US population has a diagnosis of bipolar disorder. (In a wider study including other countries, the average is 2.4%) I have no reason to doubt this because I have far too many people I knew before my diagnosis who have since told me they have an extended family member who are bipolar. And not the “oh they have a diagnosis” kind, but people who destroy their own lives and those of the people around them. I was shocked how common it is.

And this is just bipolar. There are many other mental and physical problems that affect the ability to have discipline if they just worked on it.

Edit: I should state I definitely believe focus and attention is a skill, but you can’t effectively use a skill when your tools are broken. Good luck racing a car that’s disassembled in the garage.

It's a similar thing with dieting, the number of people who have gone to the trouble of actually counting every calorie they eat, restricting themselves to a certain amount to lose weight, and not lost weight is pretty small. But the number who tell themselves that they've done it is much higher.