I never understood why software estimates are so bad when other fields do so well. When a contractor gives you an estimate of how long and how much it's going

I never understood why software estimates are so bad when other fields do so well. When a contractor gives you an estimate of how long and how much it's going to take to do a remodel, he's invariably on time and on schedule, right? And when Boeing spends billions of dollars on a new plane, they have on ready and on budget, right? So why can't software people do the same? Oh wait, complex, badly defined projects tend to run late and over budget. It's not that complicated. Spend 3-6 months defining all the details of your new web application, promise not to change anything on the fly, don't ask us to make it work on IE 7, and by the time we do 3-4 of these, we'll be able to give you a good estimate.