Why do smart people consistently underestimate how long their projects take by years, even up to a decade? You would think they would have realized this after their first project took 5 years longer than anticipated, yet they keep making the same mistake.

I'm reminded of The Programmer's Credo:

"We do these things not because they are easy, but because we thought they were going to be easy."

What is it exactly? Cognitive bias? Suspension of disbelief? A lie they need to tell themselves to keep going? Maybe most projects wouldn't be started in the first place had they known how long it would have really taken.

A good heuristic: take however long you think a project needs to complete, and multiply it by at least 3. Reality is almost certainly many multiples from the original prediction.