Very often the original system was hacked out in a couple of days/nights by one or 2 heavily-medicated (caffeine, alcohol, whatever) people.
It more or less did the job even though it has one or 2 really awful (but infrequent) bugs and is difficult to maintain or expand.
Then one day someone decides it's time for the Second System (see Fred Brooks).
They put together a team and a schedule and - like as not - spend a lot of money on fashionable faddish tools and consultants and a lot of time coming up with bells and whistles (a/k/a the Second System Effect - see above).
65 days into the 90-day schedule, they realize that they don't have any actual working code, just an immense collection of stick-figure UML diagrams, panic and put all the programmers to work coding on 100-hour weeks.
Guess what the end result is? Hint: read the papers. Somewhere between 66 and 75 percent of all major software projects fail.