I think one of the keys is to make the consequences for withdrawing obvious and high. As has been mentioned, you need to know what the PCs' goals are, and what they value. What do they want to accomplish, and what do they want to protect?

Even if the only thing they care to protect is "themselves," you can demonstrate to them that their lives get harder to protect as they cede ground to the threat.

"Make it personal" isn't always good advice, but if the PCs are truly unmotivated by anything external to themselves, you need to make their sense of "self" be on the line. Don't have evil doppelganger twins be a distant threat: they're actively seeking to replace the heroes' lives. If that isn't enough, the doppelganger twins need to steal crucial pieces of gear from the PCs to achieve their goals. Whatever of those attempts succeed will motivate a lot of players and their PCs to go after the bad guys to get their stuff back.

When they back off from a barroom brawl, have rumors spread that they're cowards and have toughs look for excuses to intimidate them. It's IC for the thugs: they feel like big and powerful people by making others back down before them. And the PCs are "safe marks" for this, due to their rumored cowardice.

If Eragon had not decided rescuing the elf-girl and avenging his father were important to him, he could have taken his dragon into hiding and gone on the run from Galbatorix...and the plot still would have had him chased down by the bad guys. Things he values would be taken from him and he would be driven from hiding place to hiding place, because that's what the bad guys do.

Even if Aladdin hadn't crushed on Jasmine at first sight and gotten involved with rescuing her, Jafar's spell would have divined him to be "the diamond in the rough" and he would've sent the guards to capture him anyway.

If Starlord stopped his various jobs and grifts and explorations, and played it safe by trying to hide out on an empty planetoid, he'd eventually starve, freeze, or suffocate to death. He has to make a living somehow. And keep his ship running. Given GotG2, even if he'd retired to Earth and taken a very safe job running a gas station convenience store, and even if he'd just played it safe with all the threats that came to Earth and made sure his ship was never found, he'd have had his Dad show up to drag him to adventure.

In short, I think you're looking for the trope termed, "The Call Knows Where You Live."