The Hard Problem
Investing for retirement is straightforward: Place as much in equities as is comfortable, contribute regularly, keep costs low, and let time do the work. Investing during retirement is far more complex. Additional considerations include establishing the withdrawal rate, determining the time horizon, and deciding how flexibly to manage both the portfolio and withdrawal rate in response to stock-market meltdowns.
Computers are untroubled by such complications. A routine solves the equations, and out spits an answer. The difficulty lies in making the answer feel real, as opposed to a parlor trick. How was that figure generated? What are its limitations? The computer is silent on that subject. It can do the math, but it can explain neither the process nor the result.