Because many forms of human social norms include a “private but not strongly so” register with neither the friction and psychological barriers of ‘true’ secrecy nor the overt exposure of publicity, and in fact a huge amount of communication takes place in that register, such as most conversation among groups socializing in public spaces.

In the digital world, this hits a wall of “the security mechanisms don't integrate with the nuances of human social understanding directly” and gets split off into “posts that are technically public but not really addressed to the public” and “invite-only areas which would probably be okay being somewhat more open but lack a good way of moderating this without leaving things open all the way”.