I have never figured out how to build a community from scratch. But I have belonged to several communities over the years which were built from scratch, without "seeding" it. I also have a track record of tremendously increasing traffic and membership for established sites. Since I have done that, both as a moderator and as a member, I suspect that statements like yours fall in the category of "I do not know how, therefore I think no one knows how and I also think it cannot be figured out by anyone." Maybe you can't figure it out. But since forums do exist which were developed from scratch, I don't think that means no one can figure it out.