There's a less cynical answer: it's that managers are subject to the same principle, of insufficient R&R resulting in tunnel vision, and so they're exhausted and hence tend to think short-term (beat this unrealistic deadline now!) instead of long-term (we'll cut the least important features or slip the deadline so we can get it done). The only way to break the cycle is to realize that a good portion of what you do doesn't need to be done, and ruthlessly cull out nonessential tasks so that nobody does them.