Because securities companies are high-risk industries.
Securities companies themselves need a lot of money and hold a lot of cash, funds, stocks, bonds and other financial institutions such as banks and insurance. If this securities company goes bankrupt, it will implicate other securities companies and trigger a financial earthquake! ! ! ! ! !
To prevent shareholders from asking securities companies to provide guarantees for their shareholders or shareholders' associates in order to defraud the company's funds, and finally shareholders will transfer funds or flee abroad, and ultimately it will be shareholders, other securities companies and countries that are unlucky.
Too big to fail, once it fails, the consequences will be very serious.
Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.