imperialscum said:

The whole back story is dumb. If Obi-Wan had any serious plans with Luke, he would have raised him himself and, more importantly, started his training at the very early age.

I have two explanations for this thing. One is simply being a plot hole. The other is that Obi-Wan and Yoda knew that Luke could never defeat the Emperor and therefore they planned to use Luke just to trigger Anakin's return who would then destroy the Emperor (which is what eventually happened in ROTJ).

It kinda makes sense. Maybe he wanted to avoid him becoming arrogant because his powers manifest to early. If he was just a regular dude who discovered the Force all of the sudden he would have more appreciation for it.

Instead of growing up an knowing that he was special from the beginning.