As of 2024, there are 20+ mainline Call of Duty titles (not counting expansions, mobile, or remasters). Three structural reasons:
Activision mandated a new CoD every November. Three studios rotated (Infinity Ward, Treyarch, Sledgehammer) on a 3-year dev cycle per game.
CoD generates $3B+/year. Annualization maximizes revenue, keeps player base engaged, satisfies shareholders. Quality varies (e.g., MW2019 acclaimed, Vanguard criticized), but the model persists.