Totally agree with the linking of the (romanticized) American frontier and the adventuring world. And arguably, PC motivations can be divided between the modes of Conan ("I know this: if life is illusion, then I am no less an illusion, and being thus, the illusion is real to me. I live, I burn with life, I love, I slay, and am content.") and Lord of the Rings ("I do not love the bright sword for its sharpness, nor the arrow for its swiftness, nor the warrior for his glory. I love only that which they defend."). Between the mercenary and the heroic.
For my part, I think murderhoboism comes particularly from thinking of the PC as a playing piece on a board game, rather than as a fleshed-out character. Bonds, Ideals, and Flaws get forgotten the moment they are inconvenient to the player's goals.