Pathfinder – I have trouble getting my players to do things in a sandbox campaign

Ok, so I'm not a GM novice, I'm a GM for about three years now. I have been leading this group for almost two years next month. I therefore have trouble motivating my players to move to action. This is a sandbox type campaign and they are currently looking for clues on BBEG's old haunts to get clues about who he is and how he was defeated to defeat him. The players have somehow imagined that BBEG wanted to kill them, this is not the case, because if it was BBEG, they would kill them and that they could not do anything about it, d & rsquo; Where they are looking for old haunts to find ways to defeat him.

So I tried to prepare side quests where they would become heroes, what the group told me they wanted to be, to help them gain power and to acquire the Experience, they even have mythical ranks. However, whenever I introduce them to something that is not just a monster hunt or murder mystery, they say, "Not at all, are we involved, that's not our problem." Until now, the only way for me to convince them to do something heroic is to use the paladin's code to make it try to help people and save lives. On top of that, I create villains and villains for the party, preparing them for awesome, and they retire thinking it's too dangerous, and again, it's not their problem. I've tried using money to pay for them, I've tried to use their character motives, but they seem intent on ignoring the world outside of the world. "main quest", in a sandbox type campaign because of something that has not happened once since. its introduction and there was no reinforcement of the game.

If it helps, the party is level 5, mythical level 2.

TL; DR: What method can I use to encourage my group of players to explore secondary content so that they are not heroically prone to heroic exploration so as not to be defeated in the battle against BBEG?