Why is cost of equity written as Ke?

Ke is shorthand. The letter k is a common finance symbol for a required rate of return or cost of capital, and the subscript e stands for equity. So Ke reads as the required return on equity. The same pattern gives Kd for the cost of debt.

You will also see re, Re or rE, especially in newer textbooks that use r for any rate. They mean the same thing. In CAPM notation, Ke = Rf + beta x (Rm - Rf). In the WACC formula, Ke sits next to Kd and the tax rate: WACC = E/V x Ke + D/V x Kd x (1 - T).

With a 5% risk-free rate, a beta of 1.2 and a 5% premium, Ke is 11%. The CAPM guide and the Excel guide use this notation, and the cost of capital entry covers the related symbols.