In Star Wars: The Last Jedi movie, we saw that
the Force Ghost of Yoda had god-like superpowers. He could not only interact with real world, he could actually create apocalyptic mayhem if he wanted. He burned down an ancient Jedi tree using lightning from a thunderstorm he created.
Given that Sith don't seem to have ability to be the one with the Force to counter Jedi Force Ghosts, why do Jedi Force Ghosts let bad guys win? Why don't they destroy ships and super mass destruction weapons (like Starkiller) before they can do big damage to the universe?
If Yoda wanted Rey to be a Jedi in the end, it means that he was interested in the affairs of the living. Why couldn't he finish what he wanted Rey to finish? Even if he had a busy schedule in the Netherland of The Force to not become a full-time Dead Jedi Master in the living world, he could occasionally interfere with the living world during big events.