A few reasons: 1. Developers do not consider unknown unknowns 2. Developers are generally asked to estimate aggressively. I have literally been asked "can we

A few reasons: 1. Developers do not consider unknown unknowns 2. Developers are generally asked to estimate aggressively. I have literally been asked "can we get more aggressive with these estimates" that were already aggressive of course. 3. Most projects require coordinated efforts across teams and almost no one knows how to estimate the other team's jobs and schedules.