Kahneman and Tversky explained that people have a tendency to disregard historical data when making predictions. Instead of forming estimates based on historical evidence (it always takes a month to paint a room), we focus solely on the upcoming task (this room is small, so it won't take long to paint).
Kahneman later expanded on the original idea in his 2011 book Thinking Fast and Slow. In it, he argues that estimation mistakes can usually be attributed to two key factors:
Failing to consider how long it's taken us to complete similar tasks in the past
Assuming that we won't run into any complications that will cause delays
Why We're Bad at Estimating Time (and What to Do About It)