The degree to which any medical diagnosis exists is on a spectrum. A cancerous tumor obviously exists, high blood pressure exists but is relative, schizophrenia - and psychiatric diagnoses generally - exist but can be more difficult to observe externally.
The behaviors associated with ADHD are so common and relatable to most folks that the diagnosis seems less legitimate. To my knowledge, you cannot scan the brain of someone with ADHD and point out an abnormality associated with the disorder. Yet we give 8 year old boys stimulant medication because they can't sit still in a classroom for hours a day.
Fwiw I have ADHD and take stimulant medication daily. But I also understand folks' resistance to accept ADHD as being as legitimate as other medical diagnoses.