Maybe he could. It’s unclear what powers a Force spirit has. But in both of the fights between Luke and Darth Vader, there’s more on the line than just winning the duel. There’s also the war between the light and dark sides of the Force going on within Luke’s heart and mind. In the first fight, it’s Darth Vader tempting Luke to join the dark side. In the second fight, it’s Palpatine. That battle cannot be won by anyone but Luke alone. Interfering in the physical fight could potentially be disastrous for the spiritual battle going on inside of Luke. If so, there’s no way that Obi-Wan would risk it.
Many fictional stories have inconsistencies to help the drama work out. To enjoy them you have to exercise a suspension of disbelief (as in intersteller space travel).
Obi Wan taught Luke the Jedi skill of communicating with dead friends. Obi Wan had learned this newly discovered skill from Yoda at the end of Episode 3.
Presumably, if Luke used it in a fight with Vader, Vader could learn the skill, and communicate with dead Sith, which would be horrifying to the Rebel cause.