Why buy house when you can build one for 15k

One night when I was browsing through Netflix and came across this movie called "Tiny" and now have a different outlook on buying houses. Why pay a mortgage on a house for the rest of your life when you can build a house the fraction of the price and move it whenever you please. These folks have have left there nice 4 bedroom homes with pools in the backyard to living in a tricked out one bedroom shack. Saving tons of money and enjoying. Only down fall is that if you have kids it would be an inconvenience and you would have to ask someone to park on there property. If im not mistaken, from what I researched the project cost around 15k to build the small house. Check out the movie its really cool.