They are already under contract when they film, and the homes they show them (other than the one they buy), are not necessarily something that they looked at. They sometimes how homes that were not in their price range or some how didn't fit their needs in other ways, just for the show.
Some people don't want new construction. Our house was new construction when we bought, but our next house will likely not be new construction. This time around we want something with more character and history.