Why did the shoguns never try to make themselves Emperor?

…which would be basically what happened in China and most other places most of the time

If we are to believe Karel Van Wolferen in The Enigma of Japanese Power, it is because having a powerless figurehead goes right back to the beginnings of recorded Japanese history.

It occurs to me that maybe they were satisfied with the next best thing, which was to marry their daughters into the family and so ensure that the Emperor had at least some of the same blood (is that what the NHK jidaigeki drama Atsuhime is about?? I find it so dull that I lose concentration after 5 minutes)