If our society needs 1% of the population to be accountants (to pick an example) but only 0.1% of the population either have incredible focus abilities or don't find accounting brain-meltingly dull, then at least 90% of accountants are going to feel like they have a lot of trouble focusing at work. Once enough start medicating (legally or otherwise) it's gonna feel to others like they really do have a condition that most don't, but they both kinda do (in a practical sense, they do need to focus better to keep up with their peers) and kinda don't (in that it's sort of our society that's sick, not them—they're just acting like most people would, in that situation).
The marshmallow test was so appealing because it was incredibly easy to perform and seemed like it was pretty close to a measure of the kind of self-control and discipline that's needed to succeed in a variety of life's most important challenges.