Why the increased use of debt increases the financial risk of the equity shareholder and hence the cost of equity increases?

Increased use of debt amplifies financial risk for equity shareholders because debt obligations must be met regardless of a company's performance, leading to higher volatility in earnings and cash flow. This heightened risk makes equity less attractive to investors, who demand a higher return to compensate for the increased uncertainty associated with leveraged firms. Consequently, the cost of equity rises as shareholders require greater compensation for the risk they undertake.