I think its largely a combination of "shiny things" syndrome, procrastination, and estimation being hard. You will have a tendency to want to do the shiny thing

I think its largely a combination of "shiny things" syndrome, procrastination, and estimation being hard. You will have a tendency to want to do the shiny things first because they are new/interesting, you will tend to put off the nebulous/harder bits, and you will not realize certain things are much bigger than you thought. Procrastination is the biggest one for otherwise well-meaning/smart people in my opinion.