There is a huge element of hubris at play. Founders are highly competitive and high achieving people, who imagine their teams to be like leading NFL football teams where there is no room for members who are not absolutely dedicated. Working a 40-hour work week is not compatible with that. As a result perhaps these kind of companies are self selecting, and it's rare to find a leader who is willing to say the equivalent of, "hey, you know, it's OK to be a loser team and not work 100 hour weeks"