I think project planning, money management, diet, and exercise are all a lot alike. Strategies for doing any of them well are mostly common sense.
Everyone knows that eating fewer calories and exercising more is better than not. Everyone knows they should save more for the future. Everyone knows they should just do the things on their task list instead of doing something they'd rather be doing in the moment.
Everyone—everyone—knows all of these things. But everyone—everyone–is only good at doing these things well for short sprints of time before their irrational sub-minds mutiny (again). And again.
Why are we so fucking stupid?
We're not really. We've just been grading ourselves using the wrong standardized test metrics. "Common sense" approaches falsely assume that people are closer to clockwork than orange. We aren't machines. We're mostly emotional artifacts of a past when the future was so improbable that it didn't make a hell of a lot of sense to waste time planning for it.
Though we are the supposedly self-aware species on the planet, we still have a long way to go before we really figure ourselves out. Fortunately the field of behavioral economics is putting us on a better course.
One of the most interesting results I've seen in the last few years came out of a study lead by Hal Hershfield. He found that people make better—and more committed—decisions about retirement planning if they are shown hypothetically aged images of themselves. He found that when we think about our "future selves," our brain activity is essentially the same as when we think about other people. So this "aged self" hack was a way of making the mental image of our future self more personal to us.
Of all the write-ups on Hershfield's findings, I like Alisa Opar's the best:
It turns out that we see our future selves as strangers. Though we will inevitably share their fates, the people we will become in a decade, quarter century, or more, are unknown to us. This impedes our ability to make good choices on their—which of course is our own—behalf. That bright, shiny New Year’s resolution? If you feel perfectly justified in breaking it, it may be because it feels like it was a promise someone else made.
Even though Hershfield's study was done specifically in the context of financial planning, I don't think it's that much of a stretch to hypothesize that this same sort of logical fallacy plagues project planning. To me, it's a very rational explanation for the irrational self-abuse we impose by giving our future selves insanely numerous and complex instructions via task management systems.
If it's human nature to feel better about dumping crap on someone else, there's little guessing left as to why so many things we plan for ourselves never happen.
By the way, if you're interested in more conversation about the future self problem, listen to David McRaney interview Elizabeth Dunn on the You Are Not So Smart podcast.