It's cheaper because it's prefab and because you need to buy the land, as stated by others.
Does that also include the costs of all permitting and taking care of hooking up to the grid and to sewers? Also, do you have to be your own general to get it done? That's how some places like that work.
It's also much more difficult to finance a prefab house. Both for purchase and for refinance. That doesn't make it a bad house, however it can make it very difficult to refinance in the future, get cash out, or sell the place.
Typically if you're financing the construction, most banks want to see stick built on lot. Not prefab.
So, there are alot of things that make this not quite as good of a deal as it seems on the surface.