You also have to factor engineering time into the plan. If you read Industrial Megaprojects, the author shows that projects which don't do a bit of upfront estimating and research tend to go off the rails on a scale that software struggles to match.
If you make a blueprint and give it to 2 construction companies to build the building, you'll end up with 2 very similar buildings.
If you give a software spec to 2 separate software companies you'll get 2 completely different programs.
You can't create a blueprint/spec that is sufficiently detailed so that 2 different companies/teams will build essential the same software.
A blueprint/spec that detailed would be the code itself.
The closest analog to software would be designing processes, or maybe designing a factory or assembly line. And the analog to the code in that situation would be the factory or assembly line itself.
It's that professionals perform similar tasks very often. Including us.