Because they want to gatekeep it and how you are supposed to act with it.

Boy, the unsubstantiated claims in this thread are really ramping up. Who's the 'they' in this case? You saw a person gatekeeping something one time so now it's some kind of problem to discuss real symptoms that impact people's lives in a very tangible way?

You should choose what is right for you and accept that others won't like it, like anything else in life

Because attaching to an unstable spectrum of symptoms and making it an essential cornerstone of an "I" is a terrible idea, no matter who you are. The "I" is just as formless, slippery, and unstable.

Secondly, when we do attach, we now have a shield we can use to abdicate responsibility for, well, the duties of being a human in a society. I've had to remind people who have thrown their hands up to exclaim, "I have ADHD!" that hey there buddy, so do I, but I still have to live my life right here in the same society/job/relationship they do.

Where diagnoses like ADHD are useful is as frameworks to understanding your own symptoms, but there's no reason to pull them in as part of you, and I've seen a great deal of harm done in our generation by people insisting on playing out their identity fantasies.