After decades, why haven't programmers figured out a way to accurately gauge the time it takes to complete tasks? Even the Mac OS X Leopard installer is horribl

After decades, why haven't programmers figured out a way to accurately gauge the time it takes to complete tasks? Even the Mac OS X Leopard installer is horrible about it. Starting every installation by saying it will take 2.5 hours is stupid when it really takes thirty minutes. Also, because no one would ever need to start a copy that would take a year, why does the Vista copy timer even go that far? It should switch to "calculating" or something more friendly and less absurd.