Perhaps the guaranteed payments of an annuity exceed the guaranteed returns of an investment portfolio. The companies managing the annuities have the capital to invest in businesses and ventures that a typical person can't. Such as financing, large real estate developments, or investments in startups, small businesses, or private equity.

With a stock portfolio, the only guarantee cash-flow is dividends since capital appreciation varies a lot year-to-year. Dividends are usually in the 2-3% range. A large, diversified real-estate portfolio might see 8-15% yearly returns in cash, which can be distributed, saved, or reinvested as needed.