People keep repeating that, without any empirical evidence at all. If it were only software developers justifying a body of knowledge, it would be ok, but all kinds of people keep repeating that.
Yet, that kind of planning predictably fails every single time in every single engineering practice.
I can guarantee you, if an engineer designs the structure of a bridge, send it to a crew to construct, and go away to only test after it's done, the bridge will fall down before it's even built.