Luke being away though was essential because without him, the Force itself is dormant. It's what makes Rey's "awakening" - sorry, the titular awakening of the Force that is Rey - so powerful. Like Kenobi, who is childless the entire saga, Luke goes from Hero to Mentor in this trilogy. You have to assume the focus is not on the Big Three of the OT and a thousand terrible novels since. Luke's incidental. He's literally the Truth and Treasure they seek. He will mentor Rey, but he won't be the hero of Episodes VIII and IX. EDIT: And basically Han says that Luke's closest friends speculated he was looking for the original Jedi temple. It's their best/most educated guess. That's all.