I don’t think it’s so much that they’re out of ideas but that they’re too scared to risk new ideas. These games are so expensive to make now so they squeeze out as much as they can with MTX and season passes to stuff their pockets as much as possible. The longer they can keep you playing, the more money they can make off of you with minimal additional effort. Wrapping up that plan with a nice widely-recognized Call of Duty logo makes them a lot of guaranteed upfront cash.

Doing the same thing for a new unknown property won’t have that guaranteed audience, will require a lot more marketing effort, and possibly years of underperforming results before it starts becoming worthwhile. And even then it’ll be impossible to catch up to the behemoth of the biggest shooter franchise in gaming history. So it just makes financial sense to keep remaking and rebooting the tried and true. Their CEOs and shareholders demand it and their audience (for now) is happy enough to keep it that way with their wallet votes.

tl;dr With development costs growing exponentially, publishers looking for the easiest path to the most profit, and gamers already primed to keep buying the next installment no matter what, sticking to old ideas is what makes the most sense. Goes for movies too.