Several reasons. The first of which is that they don't. The prevalence of ADD is something like 9% in children. Which, IMO, is definitely on the high side.
The first problem is that parents want kids to behave like adults. This means if they can't sit still, if they jump around, if they have too much energy... they immediately freak out and take them to a shrink. Shrinks get paid money to prescribe meds, and the entire pharma industry is a racket right now.
Kids are also intaking about 50x more sugar than they did in the 1980s and prior. I mean, I had a lot of candy when I was a kid, but it was moderated by a lot of other stuff that didn't have sugar in it. A grande frappe from Starbucks has like 80g of sugar in it, which is absurd.
Kids are exercising less, recess has been axed at a lot of schools, and they're getting more homework now. More hours of the day spent cooped up -> more of a coiled spring.
There's also a good deal of fear-mongering at work. Parents freak out that their child "might not be normal" if they see any kind of unusual or risky behavior and immediately start self-diagnosing, which leads to an unhealthy feedback loop.