I understand and do find it a bit....strange that the end of the evil Emperor was a fall down a shaft. As to why the Rebels didn't come in and drop him down a shaft, they'd have had to get to him in the first place, through legions of Stormies, past Vader, and then get hold of a crazy, Force Lightning shooting Emperor. It would have been pretty close to impossible for anyone other than Luke to get that close.
This.
I do find it somewhat humorous, now that you mention it though. As for Voldemort, well, he can fly so pushing him off a cliff doesn't really work.