Because those comments made me realise that what I originally wrote was (1) not quite right and (2) not perfectly clear. The extra paragraph I added, so far as I can tell, makes just as much sense if you haven't read those comments; I just wanted to acknowledge that it was other people's comments that led me to add it.
Given the choice between a discussion where some bits have been clarified in the light of later comments, and a discussion where some bits are needlessly confusing or wrong, I choose the former.
(I might take a different view if there had been lots of replies, which might have been invalidated as as result of the edit. But there weren't; just those two.)
Why not respond to those comments in child comments rather than in the parent? Surely that makes the most sense? It's very difficult to read a discussion where I have to read responses to comments I haven't read yet.