Madden - 34 years old
Pokemon - 26 years old
FIFA - 29 years old
Mario Kart - 30 years old
Call of Duty 20 years old this year! (23 if you count back to Medal of Honor)
God of War - 17 years old
Gran Turismo - 26 years old
https://www.statista.com/statistics/1285658/top-ranked-video...
I don't have experience with every franchise here but at lest with God of War and Call of Duty, there are lots of gameplay and graphical innovations that get made despite the game being largely the same at the 10,000 foot level.
On top of that a lot of people enjoy these games and are willing to pay for a version that works on modern platforms with higher res textures/models and quality of life improvements.
Mission Impossible and Fast and the Furious franchises are approaching 20 movie sequels between them, but people are willing to pay to see them because its fun and each movie brings something new to the table. Some Anime franchises have hundreds of episodes.
It's like ordering a burger or Mac & Cheese. You like it and you know what you're going to get, and sometimes that's what you want.
Any true novelty is likely to come from a mid/indie developer.