Because ADHD “looks like” moral failure. We’re lazy (actually we’re stuck in whatever we’re doing because inertia and hyperfocus). We’re careless (actually we’re distracted). We’re stupid (actually it takes us longer to process information than one would expect because of how our brains work). We’re unmotivated (well, sometimes we are, but often it’s that inertia and hyperfocus messing us up). We’re self-centred (actually we’re late because there were fifty billiion things that kept catching our attention as we tried to leave for the appointment). We’re thoughtless (actually we’re, again, distracted). We’re disruptive (can’t really argue with that one, but it’s usually not something we’re doing on purpose).
Because ADHDers usually look just like neurotypical people, and because we generally talk the same and act very similarly to neurotypical people, it’s really really difficult for them to even begin to consider that maybe we aren’t morally corrupt and actually have a really legitimate reason for all of the things we do “to mess up their lives.” This leads to all kinds of nasty stereotypes, and that has far-reaching implications: not only do neurotypical people believe and spread all kinds of falsehoods about what ADHD is, many ADHDers believe it as well. So there is resistance to getting diagnosed or revealing a diagnosis.
Add to that the fact that the current best treatment we’ve got is medication, and the distrust many people have of the pharmaceutical industry, and… well… you see where I’m going with this, right?