The thing to me is, there are a wide variety of personality traits, abilities, and characteristics that are within the range of normal.
I am kind of scatty at times, I can mult-task and pick things up very quickly but have a hard time focusing on boring tasks, I get bored easily. This has of course affected my life, some things I thought I wanted to do as a young person I realised I was ill-suited for, in a few cases I even just failed at them.
My sister, on the other hand, is good at a lot of the things I'm bad at, she isn't quite as creative in her thinking but she can apply steady work to things in a way I can't and on many tasks can accomplish more. She's done well at things I'd struggle with.
I have a cousin who is quite slow, and can't manage many of those kinds of things but she is cheerful and good with people. It took her a while to find a job that suited, many she couldn't do.
Sure, you could do brain studies or whatever and perhaps find out why we are all differernt. But that's not a diagnosis, because none of us are ill or have a condition, we are just differernt people with differernt personalities and abilities.
So many people now seem to think that their personality requires some kind of diagnosis.