It Makes No Sense

I have a question that I am hoping someone can answer. I recently watch
Episode III and I don't understand a few things at the end of the movie. Why did Yoda and Obi-wan go into hiding? Neither one killed the Sith they went out to kill and they both decided to hide instead of lead the rebellion against the empire. It seems to me that with two of the most powerful jedi that the rebellion would have stood a chance and been able to mount of fight, instead of having to wait 20 years or so until Luke was able to fight. What good did Yoda do anyone on the planet that he went to. He didn't do one thing, in fact it seems that he probably went crazy. Secondly, when Obi-wan took Luke to Tatuin why did he never speak to Luke, and why did the uncle refer to Obi-wan as some lonely hermit? I don't understand why Obi-wan wouldn't have trained Luke from a young age. I hope someone can answer these question because my finite brain can't comprehend them.

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