Like most of the problem with our society, it's people knowing one thing about a person and imagining them as a charicature based on that, rather than seeing complete, individual people.
They hear 'benefits for ADHD' and think of a stereotype - someone gaming the system, loudly proclaiming their kids' diagnoses for attention, sitting around watching TV all day simply because they don't want to work, etc etc. "We all know people like that" - maybe you do, or maybe you're only seeing part of the picture.
Two examples.
A good friend of mine has moderate ADHD. He can't hold down any kind of structured role - he's very clever and likeable but doesn't get things done because he has no concept of a deadline and has emotional outbursts when overwhelmed. He's ex-army but never got far up the ranks for this reason and has the usual additional challenges of ex-Forces personnel in the workforce. He does now work in a self-employed capacity, and is very good at it (he's fantastic with people, caring and very creative) but only because his wife helps him with the admin and time management.
I keep in touch with a young lad I tutored years ago, now in his 20s, who has more severe ADHD (alongside dyspraxia) and can't work. Or, more to the point, can't hold down a job. He's not safe in a manual role due to the combination of dyspraxia and complete lack of danger awareness, and he doesn't last long in non-manual roles because his lack of executive function means he doesn't get tasks done independently. He will quite literally wander into a building site without a helmet and have no idea he's doing anything unsafe until told. He's unable to focus - it's not a choice he's making.
I'm not saying there aren't people taking the mickey. Wherever there's something for 'free', there's someone gaming it. But the mass mockery of absolutely everyone with ADHD needs to stop. It's funny how if there's something wrong with your heart, it's seen as serious, but if part of your brain isn't adequately functioning it's seen as your fault.