It's a nice observation that Spec-Driven Development essentially implements the waterfall model.

Personally, I tried SDD, consciously trying to like it, but gave up. I find writing specs much harder than writing code, especially when trying to express the finer points of a project. And of course, there is also that personal preference: I like writing code, much more than text. Yes, there are times where I shout "Do What I Mean, not what I say!", but these are mostly learning opportunities.