@Suzette, you had me thinking deeply about this one. That's an interesting question. Apologies in advance because these aren't fully formed thoughts but I think this could be related to autism. I'm reflecting on the fact that people on the spectrum, especially as children, can engage in black-and-white thinking. It's either this or that. You're organized or you're not. Additionally, given some of the issues with theory of mind, maybe you observed someone being neat/organized and saw that as a personality trait inherent to that person and not a choice.
I also think people on the spectrum can struggle with identity formation. One way we may understand ourselves is in relation to others. So when you saw someone being neat, it fed into your identity as someone who was the opposite without occurring to you that you might be able to adapt.
That said, this is still a difficult concept for me to grasp. I was just telling my Mom how I appreciated that on two separate occasions, when I've talked to an autism clinician about my ADHD/disorganization they've said, "You need to hire an assistant!" They also suggested some practical steps I could take but there didn't seem to be this pressure for me to be someone different. It was as if they started by accepting the fact that I was disorganized and then gave me suggestions on how to be successful
as a disorganized person.
I look at "early birds"/morning people with envy. I wish I was someone who could hop up out of bed at 5 AM and get to work. I'm the complete opposite. On the weekends, I'm lucky if I'm rolling out of bed by noon. Like you, it never occurred to me until fairly recently that I could change my behavior and perhaps turn myself into a morning person. At some point, I decided I was going to force myself to become one. However, studies show that people naturally have different circadian rhythms and that those on the spectrum specifically, are more likely to have circadian dysregulation. I've read that attempting to sleep/get up earlier or later than is natural for you might actually be harmful to your health.
We're also more likely to have executive dysfunction which might impact things like neatness and organization. Those of us who struggle with this can certainly learn strategies and habits to help mitigate the chaos. However, I feel that I am not the most organized person and never will be. When I try to be super neat, I feel it can produce anxiety. Recently, I was trying to organize my room. It started off okay but at some point, I was literally pacing back and force, anxious and unsure of what to do next.
So while I don't want to say that some of us are just doomed to be disorganized and messy, I'm not fully convinced that your thought process as a child/young adult was that far off the mark. Maybe you're simply not a naturally tidy/organized person like some of the people you observed. You may have just lacked awareness of your ability to mitigate the chaos.