Hello Kristina

This is a good question!  I have written about this in my post, "Why Some People Don't Believe in ADHD."

I think part of it for some people could be their age.  Decades ago, ADHD as a diagnosis did not exist.  When I was in high school, for example, the diagnostic label  of ADHD was just being included in the Diagnostic Statistical Manual.  So it may be hard for some people to accept that this is a real disorder.

Another reason may be that ADHD is difficult to diagnose.  There is no single gene to identify someone who has this.  There are no physical characteristics to provide a definite diagnosis.  What is relied upon are behaviors...which some say are very subjective. 

So these are but a few reasons.  ADHD is one of the most researched disorders and also one of the most controversial...many people still argue about its existence. 

Thank you for your question.