Not saying you're wrong in all cases, but there are enough examples of hugely expensive megaprojects which totslly tanked, which would have definitely been much more successful with OPs approach if executed correctly. Not saying they would be done and done within a weekend, that's silly. But the alternative, poorly defined integration interfaces, multiple contractors, multiple stakeholders with conflicting requirements and zero (real) regard for the user is unfortunately fairly common, both in public (city/regional/government) and private bureaucracies.

The examples are legion, and they always seem to have NIH and baroque requirements, and be rather over- than underspecified. I would go so far as to say that these projects are almost never successful (and definitely never on time and budget).