They're small, but they can be hitched to a truck and moved anywhere. The owner of the trailer doesn't have to own the land that the trailer's on. Instead, he/she can just opt to pay rent. The price of the mobile home itself would be about 1/4th the same price of a conventional (and, to be fair, considerably larger) home. According to one source, a mobile home manufactured c. 2014 would have an average lifespan of about 30-55 years, and I will not discount the possibility of construction design improvements in coming decades increasing this figure somewhat. In any case, a person who bought their moved out and bought a mobile home of their own at 18 would find that he'd likely only have to purchase and move into a new house once more in the span of his life, barring damage incurred from close proximity to natural disasters and whatnot.
It would serve to make housing significantly more affordable in America if mobile homes became more popular, and, perhaps equally importantly, it'd allow for greater flexibility in the jobs market: in today's climate, many Americans are reluctant to move into a new house because they're not sure they'll be able to sell the old one, which would straddle them with extreme debt. So they may turn down more lucrative job opportunities elsewhere in the state/country. If all you needed was a moving truck and a new trailer park somewhere, I suspect we'd have a much more dynamic economy.
A big barrier to this is the sordid reputation associated with trailer parks: they're thought of as low-income housing units, where you'd be surrounded by distasteful and even dangerous neighbors. If the image of trailer parks could be rehabilitated so that it was a socially acceptable option for the middle class, that'd solve the problem I think. Also, the government could work with manufacturers of homes to design a model of more "serious" mobile homes which are better insulated, ventilated, and energy-efficient.