I've tried forcing it 5x and found that the fifth Why is exactly the point where they get annoyed and stop talking to you. So there may be a magic insight at the fifth Why but you'll never hear it :)
Often software is used as a means for one group inside a company to control another group.
Of course, some issues are genuinely simple, and you're off in unhelpful territory halfway through. I suspect five why's is a decent estimate, but not necessarily more accurate than four.