The reason there seems to be a rise in support groups is because there has been a rise in diagnosis. Simply due to the fact its been under or mis diagnosed for decades. There aren't many other conditions that are underdiagnosed to this level, so I wouldn't expect there to be a rise in diagnosis of other conditions. But the huge spike in ppl getting diagnosed has led to a huge demand for services.
People don't understand the condition, they don't know how to cope and it is still a very stigmatised set of conditions. Where people don't really see it as a as serious as other disabilities, this also creates a huge need for people to talk to people that do understand and don't judge.
(Im going to make it clear that the rest of this post is to educate not insult, as I genuinely believe you meant no offence when you wrote this, but it illustrates the point I will go on to make, and hopefully gives some insight into your question)
You yourself, without even realising, totally trivialised the condition (adhd specifically)as being not as bad as the disability your child has, and therefore your struggles as a parent of a disabled child were not comparable, when you said
"In contrast, other parents in these groups have the challenge of their DC having no awareness of danger and running into roads etc, yet there is hope, I presume, that their DC will learn ways to manage these impulses? Otherwise we may have adults in a few years who will be running into roads? This difference matters because there is little common ground"
What is it about ADHD in particular that makes people certain that it can't 'be dealt with' 'get better' 'just learn to not be so impulsive' 'just try harder'
No, they do not just figure out a way to deal with it, and yes, you do have and will have adults with adhd doing the equivalent of 'running into roads' figuratively and literally!! but because the majority of people think this way, that is why there are so many resources on offer, and so much of a fuss being made to raise awareness, because I'm sure you have never had someone infer that your child would surely just get over their disability as they got older and wouldn't need as much help as an adult.
The fact that there is a total lack of support for ALL disabilities is not something I would argue with at all, BUT picking on the neurodivergent population who up until now have had next to nothing, because they are suddenly being offered the support that should have always been available based on it apparently not being 'that serious', can be very harmful. This group of people are having to prove they even have a real disability, whilst simultaneously defending their diagnosis and desperately trying to navigate school ot work in a world that mostly still thinks, you could do it if you just trued hard enough. Trust me, there is no where near enough support for these people
Every person with a disability deserves support, every parent of a disabled child deserves support, but no disability is above or more worthy of support than any other. And adhd is one of the only conditions where certain people will tell you it doesn't even exist, whilst parents are judged as just being bad parents.
What other disability can you name where people will say there's nothing wrong with the child. The parents just don't discipline them properly?
Support for ADHD is also very very complex due to it being a spectrum disorder, so there's no one size fits all place to go. And pkease excuse the crude analogy here, but its just to illustrate a point For example, if you are paralysed from the waist down, you will be a wheelchair user, you can join a support group where EVERYONE is a wheelchair user, you will all have this in common and will be aware of the challenges you ALL face-theres no ifs or buts about not being able to stand or walk, you can or you can't. There is no such definite criteria for spectrum disorders because everyone's level of disability and need will be monumentally different, this is another reason there are so many groups because one size does not fit all. I hope this gave a little insight into this 'phenomenon' and once again, educate, not insult is ALWAYS my intention. And if anything I said came across as upsetting negatively impacted you in any way, I assure you it was not my intention and apologise.