Building a mountain is expensive. Making a million cars is not cheap either. Mind you, nor is building a zillion houses. This summer I stood on the edge of a mountain in North Wales just seconds from taking a zip wire down to the bottom. From the edge, I could see a great business. He had used existing assets that were largely unused and got them to do something. Building a zip-wire across a mountain costs nothing compared with building a mountain.
And in a way, he was just doing what Airbnb or Uber have done. Find assets that are doing nothing, and get them to do something. The rooms that were lying idle suddenly became full. Cars that were driving around the city empty suddenly had paying passengers in them. Neither had to build the infrastructure of the rooms or the cars. That takes a lot of money.
By comparison, coding a website to turn unused assets into used assets, well, that is relatively cheap compared to building a million cars, or a zillion rooms. Or building a mountain.