The short answer is optimism and poor assumptions. The hiking analogy works well to illustrate this answer.
Everyone who has been a part of a software project knows about poor assumptions. From the start, you don't have well-defined requirements. You don't have a complete design. You don't understand the consequences of decisions you have made or will make. Any estimate is doomed to fail in the face of all these poor assumptions.
Staggering a software project into smaller chunks goes a long way to controlling these poor assumptions and the impact to schedule and, more importantly, customer expectations.
The principles of the agile manifesto go a long way for mitigating the too common pie-in-the-sky wishful thinking that dominates software engineering. They work even better when the customer realizes they are a part of the system under development and its success - if they can keep that balanced with giving the development team the space to do what it is they do.