The lightning could have hit the tree even without Yoda doing anything, just by coincidence. Yoda slightly influencing the weather do do something that could have happened naturally seems very subtle and not too far fetched to me. Now, how do you imagine ghost Obi-Wan fighting against Vader or the Emperor? What could he possible do to defeat them?

Also, so far we only see force ghosts in calm and peaceful moments, not when people are busy fighting each other and not in the presence of a strong dark side user. Maybe Obi-Wan really wasn’t able to help Luke simply because Vader’s presence prevented him.

And finally, the main role of force ghosts is to guide others, not to solve problems for them. Yoda sets the tree on fire as a lesson for Luke, to set him straight, not because Yoda hates the Jedi books (they weren’t even in there and I think Yoda knew that). Similarly, what lesson would Obi-Wan teach Luke by killing Vader for him? Facing Vader is an important station on Luke’s spiritual journey and he needs to do it alone, otherwise it would be ruined. Isn’t that similar to LOTR, where Gandalf in theory is more powerful than Sauron and could easily kill him, but he rather helps the humans to overcome their struggles themselves?