Actually, that's an excellent point -- how do buildings/bridges/etc. actually get built? I mean, laugh if you will, but can you not think of any examples of construction projects that straight up ran out of money and had to halt? Or that became enormous money and time sinks, inching their way but never-quite-making it to completion?
As I understand it (still pretty new to the industry), we're supposed to give realistic estimates for a product's completion, considering that 1. requirements are always incomplete and 2. unexpected complications will always arise. 'Done' is a moving target, and part of its estimation is ensuring the product is deliverable at every stage (with varying degrees of completeness), regardless of whether how many times everything goes pear shaped.