Presumably this is because allocation of capital drastically augments the labor that people do. (investing in a farm allows a farmer to replace his hand-plow with a tractor, drastically increasing output). I learned Intro. Econ from Greg Mankiw's textbook "Principles of Economics" and I was very impressed by his reasoning.

If this wasn't a sincere question then I apologize: my ability to read sarcasm is limited.