If it's about 5% of people which is what the current research suggests, that's quite a lot of people - one or two in every classroom.
Go back to 1990 and there were something like 40 children in the whole of the UK on ADHD medication, which suggests that there weren't 1-2 children in every classroom being diagnosed - it was probably only the extremely severe and disruptive presentations, which are far more likely to be boys and more likely to have been picked up anyway because they end up in the criminal justice system.
The children of the 1990s (not to mention the 1980s and the 1970s) are adults now and if you had a perfect system where all (or almost all) the 5% were being diagnosed say between the ages of 5-7 then it would in theory be manageable to get through everyone. But most of us weren't - even in 2005 when I was struggling at college with what I look back were absolutely textbook ADHD symptoms and having endless meetings about it, the support offered was "Well you just need to find more self motivation and organisation". Then they withdrew me from the full assessment because it was obvious I hadn't done enough work to pass. ADHD wasn't even on anybody's radar (I don't blame anyone for this, but it is painful to think about what might have been possible had I understood my difficulties and/or been able to access medication then).
There have got to be over a million undiagnosed adults in the UK. Maybe it wouldn't cause everybody problems, but for a lot of us it does. And while there are probably also huge numbers of undiagnosed dyslexic people, autistic people, dyspraxic etc - those things don't have a promised medication which can make real difference to people.
A combination of better awareness making people recognise oh - this fits, I must have been one of those missed - and cuts to mental health services in general are just combining to make a huge bottleneck. I don't think ADHD is the only area this is happening in, but it is happening acutely with ADHD and I think these three factors (better awareness causing a catch-up effect, people more likely to seek diagnosis to get medication, NHS cutbacks) are probably the reason. There are waiting lists everywhere in Europe although not quite as bad as the NHS.
I am very grateful that I was diagnosed before the current backlogs - I feel for anyone who is caught in it.