Why isn't the surplus of donations from cooperative insurance carried over to the following year instead of requesting new donations?

What governs this matter is the contract between the donors and the company's management. Your statement that the insured is the sole beneficiary is incorrect. The board of directors of a cooperative insurance company is an agent and partner of the shareholders. It invests their funds and distributes profits according to the contract, either to the shareholders or by returning them to the fund, all based on agreement and mutual consent.