--Snip--
ADHD and ADD could, in theory, have a specific cause in the environment that has only recently occurred. I don't know for sure. But also, I know that 30+ years ago, most kids who had some form of ADD / ADHD weren't able to function in a classroom because there was no medication. Thus, they weren't in your classroom because they would simply have been too disruptive / unable to learn. They, likely, would have been placed in a classroom alongside kids with Down's Syndrome, for example, which is why you never saw them.
Also, the further back in time you go, the lower the average school leaving age. So, fifty years it was much more common and accepted for people to stop their education in grade 8/9/10, so people who had trouble coping with the incompatible and strict environment of school had a much more viable option of dropping out and simply working instead.
First, "nature" doesn't want anything. Secondly, things happen for no reason all the time. Humans might ascribe some reason to events, but that doesn't mean there was some sort of reason / reasoning behind those events. If you want the "reason" why some kids have ADHD and others don't, here is the best I can give you: the mother and father (or IVF or whatever) had such and such genetics that caused a neurological development in the child's brain which, possibly influenced by the kid's environment, caused the child to have a lack of attention, hyper-focus, etc, etc. That's your "reason."
On the talk page for Wikipedia's article on bipolar, someone once requested coverage of the evolutionary advantages of BPD. I promptly responded by pointing out that there is no guiding force behind evolution, and that there are plenty of stuff in our genetics which could easily be described as defects (numerous inherited physical diseases which clearly have no benefit). Sometimes stuff survives
in spite of their negative effects (and low prevalence indicates little advantage given; if something is helpful, it, like eyes, will eventually make its way into the bulk of the population).
And a slightly semantic point is that "concentration" is a far more accurate word to use than "attention". I have plenty of attention, its focusing it on a given task which can be a problem
.
Yea, that's how my uncle described it with the added bonus he found certain things he was hyper sensitive to and would distract him from getting concentration. Stuff like someone shuffling papers, or coughing would drive him up the wall nuts. He tried most of his life to get a bachelors in anything, but always petered out on the interest scale. He also has ADHD, and is incredibly thankful that it hasn't shown up in any of his children (and now grandchildren).
I once read something (may have been only anecdotal) about people with ADHD often being hypersensitive to various things (not all have all, but some of the stuff popping up a lot overall). As I recall, things like touch and sound were among them. It was ages ago, though, so inaccuracies are highly likely.