Because he never considered retiring and letting the voters make that decision without him on the ballot? You don't have to stay in office, Chuck.
Sen. Chuck Grassley (R-IA), 93, says he is concerned about Rob Sand (D) becoming governor: “I don’t plan on dying in office, but if I did, I don’t want a Democrat governor choosing my successor.”
VoteHub rates the Iowa governor’s race Lean D, with Sand at a 77% chance of winning.