Yes, because the federal government can't assume that everyone has an ID, since they don't issue a universal ID. Any attempt to fix the fact that Americans don't have universal federal identification has met stiff resistance from a variety of angles, from privacy proponents to religious nuts who think universal identification is the mark of the beast.

It ties into why we still have to register for the draft (despite not having a draft since the 70s, and being no closer to instituting one than any other western country), and why our best form of universal identification (the Social Security card) is a scrap of cardstock with the words "not to be used for identification" written on it.

So, there's no universal ID, it's illegal to mandate people have ID, and freedom of movement within the United States has been routinely upheld as a core freedom. Thus, no ID required for domestic flights.