Because of diminishing returns to capital. Diminishing returns mean actual investment eventually cannot keep up with break-even investment.In this video I introduce the Solow growth model and show how to solve for the steady state. Solow sets up a mathematical model of long-run economic growth. He assumes full employment of capital and labor. The document discusses the Solow growth model and how it can be used to analyze the effects of changes in saving and population growth rates on economic growth. The steady state is the key to understanding the solo model. At the steady state investment is equal to depreciation. As the capital stock increases, the additional investment tails off but the additional depreciation does not, so at some point sYt equals δK. Knife-edge case, requires the production function to be ultimately linear in the capital stock. 2.