A wonderful way to work a project is to build as you go, no deadlines, just an environment where you can do your best work, without pressure, without impacts that have nothing to do with your project, with the best resources and an unlimited budget.
But that rarely happens.
A one of the truest tenants of Agile is that the people closest to the work decide how to do the work, when it can be delivered, and how much time to spend on the work for a quality outcome. We have seen the pictures of an SUV on fire from a cell phone that got too hot. Word is the product was rushed to marked because of a competitor. No doubt the engineers were not ready but were hastened to act quickly, trim off the QA times, rationalized defect reports, and let those farthest from the work decide the work, which ended up in disaster. Always does.