And I pull out my ten-foot pole to poke it.
Because after all there are plenty of adventurers who never settle down, or who take a good long while to do it. Conan the Barbarian only did as king, which constituted two of his numerous adventures. Fahred and the Gray Mouser. Many another sword and sorcery hero -- I consider it one mark of archetypally pure sword and sorcery, that the hero loves to adventure and goes on and on having them. As opposed to epic fantasy where the hero settles down after.
It's needed in a lot of series, but it's also used in stand-alone works. How poignant it is for the character to ride off along the road, alone. . . or sailing away from the Gray Havens.
Which is one reason why it's not more common. Rootlessness is a hard condition to make an aspiration. Especially if the character is already rootless. Like knocking off the parents so the hero can act without wondering whether it was right to shove them into a nursing home, rootlessness allows the hero to move the plot like nobody's business.
So why doesn't he stick to it more often?
Well, roots and homes are indeed a common human desire. And making them the hero's desire allows him to achieve his aim. Character change! Especially if he had wanted to adventure. Furthermore, unlike an endless desire for new and different, it allows the story to end. I believe it was E. M. Forster who said that weddings and funerals are very convenient for endings -- which is true, they give shape, and finding a home can serve the same purpose. Endless wandering means endless adventuring, which makes the ending always open.
Part of bittercon