> you lay out a huge specification that would fully work through all of the complexity in advance, then build it.

I have tried this a couple of time even for small projects ( a few sprints ), and they never worked out. I'd argue it never works out if you are doing non-system programming projects, and only has a theoretical non-zero possibility to work out for system programming projects, and perhaps a 5-10% to work out for very critical and no patch possible projects (like moon landing).

Because requirements always change. Humans always change. That's it. No need to elaborate.