"decide what the software is supposed to do in the first place."

After 20 years of software development I think that is because most of the software out there, is the method itself of finding out what it's supposed to do.

The incomplete specs are not lacking feature requirements due to lack of discipline. It's because nobody can even know without trying it out what the software should be.

I mean of course there is a subset of all software that can be specified before hand - but a lot of it is not.

Knuth could be that forward thinking with TeX for example only because he had 500 years of book printing tradition to fall back on to backport the specs to math.