Personal experience: I'm in this meme for "thinking" work. For light physical work, I can work in a steady state for quite some time.

I think it comes from the possibility to go "all out" with thinking and tiring the brain very quickly. And we're evolutionary absolutely not used to that.

Coincidentally, I think most people suffering from this are also in programming or scientific work. But causation could also be the other way, such that people with ADHD tendencies choose such jobs more often.