I guess I've always felt it's psychologically more relieving and 'easier' to throw your problems or very self onto a label - whether that label be an mbti type, something like ADHD, or an adjective descriptor of yourself, like 'scatterbrained'. All labels might be true, such as they are, but the danger, and inclination, is to throw more weight onto them than truly necessary, and to start using that label as an excuse, a crutch, or some sort of get-out-of-jail-free pass to prevent yourself from reaching beyond the label or doing something that opposes the label. Or you might start seeing yourself as JUST that, so you effectively box yourself into something you've mentally created/accepted about yourself, and see yourself as incapable of anything else.
Doesn't matter the label/grouping -- as others have already said, it's something that is human - going back into history, and across cultures. The groupings just differ between cultures, but the commonality would be that there's an expectation of sorts for people not to push too far out of their label - to act out the label, perhaps. (haven't put too much thought into this, so might have to revise some of these thoughts later)
I don't know that it's tied to our culture specifically, as Ivy could probably dig up quotes from centuries ago saying the same, but I've also noticed with the onset of the ADHD phenomena (past 10-15 yrs?) and other 'disorders' really coming out into the open, that it also lifts ones own accountability for ones actions, to a degree, if you have a disorder. That might be part of the 'cool' aspect of ADHD. "Hey! I have ADHD! That's why I'm such a troublemaker/etc/etc/insert noun/adjective, but since I can't help what I'm doing, I can't/shouldn't really be punished for it. You need to account for me in a different light." Might foster that sort of attitude, but I'm certainly not trying to say everyone feels/reacts this way, or that people don't in fact need to be addressed on an individual basis, and am not trying to put down anyone who does struggle with something like this. It's just easy to abuse it - whether on an individual level, or a cultural level.
Edit: To put a more positive spin on it -- people ultimately want to feel less alone, so they tend to reach and build towards commonalities with others.