Because it’s a spectrum everyone is on. Clinical diagnosis asks “how many,” “how often,” and “is it causing problems” to locate a patient on the spectrum. So yes, ADHD patients’ experiences are indeed mostly common; it’s the rate of recurrence and their control of it that is not.

Edit: Also it’s notoriously underdiagnosed, so when I see comments saying “but that’s what it’s like for everyone” or “almost all of these memes apply to me,” honestly I’m compelled to ask how many apply, how often, and is it causing problems.