I would assume because retirement fund managers and pensions like constant safe returns on investment, given they have a to have a constant payout, they go for things like debt, especially tasty non-dischargable student loan debt. Debt is a lot easier to predict than investing in companies.
Investing in companies vs lending money is a pointless distinction here. The total return for the business venture is the same if, for example, a company borrows money to buy back stock. All kinds of the return-on-savings are fungible with each other, and work together to drive down the risk-free rate of return.
