The tax shield on debt is one reason why:

the net cost of debt to a firm is generally less than the cost of equity.

the cost of debt is equal to the cost of equity for a levered firm

the value of an unlevered firm is equal to the value of a levered firm.

the required rate of return on assets rises when debt is added to the capital structure.

the value of an unlevered firm is greater than the value of a levered firm.