This is no way disregards the difficulty of living with an actually severe case of ADHD but is not what most of these people are dealing with.
EDIT: many seem to have misunderstood what I mean by this. I'm not saying these people are only claiming to have ADHD to use it as an excuse. What I mean, is that they may very well do have, and they're using it as an excuse. Mostly to themselves.
I think people want simple explanations for the world around them but more often than not the truth is not simple. A person without ADHD can suffer from poor self-discipline. I don't see any reason why someone with ADHD couldn't. It doesn't make them immune to the other personality quirks that people commonly have. I personally just don't think that a diagnosis gives anyone the excuse to stop trying. Play with the cards you've been dealt.
Even in this thread there are gatekeepers implying that because I'm not a completely dysfunctional wreck myself I must therefor not have ADHD.