Uh, Luke?
Force Ghosts can pretty much linger around in the real world whenever and wherever they want. So, you can imagine that Force Ghosts have caught sight of some pretty X-rated stuff. For instance, the scene where Luke and Leia lock lips? Even though nobody knows that they’re brother and sister (yet), if Obi-Wan were around, we bet he’d have an expression of utter disgust on his face.
We admit that having Obi-Wan around would be a major upside. He could shove crates in front of your enemy while you’re fleeing. However, for some reason, neither living nor dead Obi-Wan bothered to inform Luke that he was related to Leia. Before kissing would’ve been the optimal time, Kenobi!