Agriculture is land. People need land to build their homes, even if they are skyscrapers that create a landscape.

We tend to think of soil (click here) as a renewal resource - one that is constantly being replenished by decaying matter. But worldwide trends suggest that we may need to rethink that common understanding, not because our scientific knowledge is flawed but because our behavior has resulted in depletion at rates tat exceed replenishment.  has somehow changed "It takes approximately 500 years to replace 25 millimeters (1 inch) of topsoil lost to erosion. The minimal soil depth for agricultural production is 150 millimeters. From this perspective, productive fertile soil is a nonrenewable, endangered ecosystem." 
Source:  David Pimental, "Population Growth and the Environment: Planetary Stewardship," December 98....

Among the focus of the 1960s when it came to Earth was realizing the limits of Earth to sustain so many human lives. I will let Captain James T. Kirk explain.