well, movie (heck, all MM movies took place in Australia, which is anything but mountainous land.

and it goes on - if rivers are poisoned, rain is some sort of hardcore acid pouring down. you actually want to avoid that as much as possible. so no, even for people that would be able to travel far, mountains might be a worse choice. plus, they are so deep into apocalypse, that they don't know geography beyond what they can reach by cars