Why are people such bad estimators?

“Painters say 5 days, it takes 8. Designers say 2 weeks, it takes 4. Programmers say 3 months, it takes 5. Government says it will take $X and it takes $XX. It’s been a very rare occasion (in my life, at least) that estimates have been anywhere near accurate. Even people in business for 20 years seem to have a bad time making accurate estimates. I realize an estimate is just a guess, but there’s a lot of bad guessing going around. What is it about people that make us such bad estimators?”