The force told them to wait. If Ben had run, hidden and been living in Tatooine all this time, and had Owen not known Ben lived on Tatooine all this time, then Luke would never have met him, and Ben would never have given him his fathers saber, and Luke would never have been able to go on and.... Had Yoda not run away, and spent all his years hiding in a horrible filthy swamp, far below his pay grade, for 18 years, then he would not have been alive to train Luke after Ben had died. Yoda managed to stay alive only just long enough to train him too. This was all foreseen, and they had to be ready. They were not cowards - they each lived for 18 years knowing every day that their friends had died, that the galaxy was living in servitude, and that they could not stop it - except to live this worthless life in squalor, in the hope that the force visions they had would one day come true and they would do their small parts.