I don't think it doesn't exist, I think it's not a useful label. Partly due to the way it's treated as a long term issue that should be addressed with drug use, and partly because it can often be unhelpful to the individual's own psychological state, some people flip into an "i'm broken" mode and stay hidden behind it rather than make progress.
There is however good reason to believe in over diagnosis. ADHD is considered a neurological disorder due to physiological differences in the brain. However it's diagnosed through observation of a combination of individually non-unique behavioural symptoms... which is why when you are suspected of ADHD, you are sent to a psychiatrist.
Regardless of whether the cause of such behaviour is physiological or purely environmental, in both cases environmental factors can have a huge impact on how it affects you, this is pretty obvious for anyone who has been in the presence of someone severely autistic. Drugs are not the answer to everything, and not usually a healthy long term solution.