I think the biggest testament to this is the 50%+ dropout rates common in PhD programs. You're starting with some of the most intelligent, diligent, bright-eyed young people on the planet and letting them work independently in a field that they presumably have a deep passion for.
All we ask them to do is write 100 pages or so on a topic of their choosing. They're given four or five years, which comes out to less than a page a week. Yet more than half will fail to complete the task due to attrition. (Yeah, I know this is over-simplified...)
And I think the obvious answer is that humans, in general, are horrendous at being self-directed on long-running, large-scale, multi-step tasks.
Almost anybody who consistently produces anything of real value works on a team, where at least there's some semblance of regular accountability to other humans. The rare example of truly productive people who can work in isolation are almost all (rightly so) lauded as atypical geniuses.