The problem is ADHD is a "trait" not an illness - it is an inherent way of being (as are many other conditions). It is not like cancer in that respect and comparisons with such are meaningless.There is a huge variability with how people cope with it: when it is lined up with their other traits this can either make it harder or easier for them to live with, to the extent that for some people it is disabling and that for others it isn't has little to no material effect.
However, we are now less willing than ever to view it as something with which many people can "cope". Yes, you might have been a chief rocket scientist for NASA without it, but if with it you are holding down a job, supporting yourself and not suffering excessively it is not disabling, and there is no need for the state to intervene. This is the same for many conditions, and coping can change. Some people will become less able to cope, others more able.
But ultimately the problem is taxes are sky-high, they are not paying for what we need already, people are dissatisfied with the state of affairs generally (I know I am) and so (a) feel they should be entitled to more and so will think about claiming when they haven't before and/or (b) will feel more resentful of people claiming. Additionally, I think we are less resilient to pain and suffering of any kind than we were 30 - 50 years ago. We are used to comfort and we don't expect things to be hard, which is a great testament to the leaps society has made, but we can't expect to be comfortable all the time.
My personal view is that if you can manage to support yourself, the state shouldn't be required to intervene; and individuals should feel a moral obligation to do everything they can to avoid state intervention. People talk about "burnout" and I get that it is much harder for some people to do certain things than others but no one has a right to live a life that doesn't render them exhausted. Would it be nice? Yes, of course, but for some of us, that is just what there is. Again, it is not the job of the state, in my view, to tax some to pay for other's comfort. For their necessities when they can't do that themselves yes of course, but so that people who can have a reasonable standard of living without assistance to have a better one? I personally don't think so. You of course then get into what constitutes "reasonable", and that is an excellent question, and one that might result in many more or many fewer claimants, dependent on where you fall. But there has to be an analysis of the cost against the societal benefit, not just the benefit to those who are recipients. I honestly don't know where I would draw that line, and if I did whether eligible numbers would go up or down. What I do know is that I have a lovely series of "alphabet" conditions/traits that have made my life less or more difficult over time, that I go to bed and get up exhausted, that I am almost always in a greater or lesser degree of pain and that sometimes I shut down completely for a couple of days to recharge. But I can manage, and so I do. It is what it is. I'm not sure why other people should pay to make my life easier when I can cope. And I think we really need people to try to cope more.
Finally, the massive uptake in claimants based on conditions like ADHD because of greater awareness means that benefits are simply unaffordable, and so that is drawing attention to it. And shockingly, the first place we look is at conditions that have seen th biggest increase in claimants.