Luffy's dream is to be the Pirate King. By definition in One Piece itself, the Pirate King is someone who most freely wanders in the Grand Line. Surely, finding One Piece is a necessity to become the Pirate King, but is not an enough condition.
Luffy loves adventure by nature. He doesn't want to find One Piece so easily. If he wanted so, he could ask Rayleigh its location in Sabaody Archipelago arc. When Usopp asked it, he shout with a great voice and made Usopp stop asking it. And he said, if he were to learn its location, or even learns if it existed or not, he would quit being a pirate at that moment.
Luffy simply wants to follow steps of the former Pirate King Gol D. Roger. He want to experience a good adventure. He wants to fight strong opponents. He always chooses the hardest path.
If finding One Piece was his only goal, he could have of course spent his time learning cliff climbing skills instead of wasting his time in the sea. Then he could finally climb that huge wall of Red Line and search for Raftel there.
One more thing to note is that the Location of Raftel is unknown. Even Shirohige didn't know it. Remember that, in a flash-back during the war in Marine Headquarters, Gol D. Roger asks Shirohine if he wants him to tell the location of Raftel.