Why are only some games allowed to do something?

Revisiting the same era is perfectly fine and it works fine for other franchises. It's about innovation and creativity and a few COD releases have been fairly innovative.

For a lot of people, the issue with some of the annual COD titles is that they didn't do anything new, they were just by-the-numbers games that were competent but unremarkable.

The first Black Ops for example was a real knock-out, even coming from a franchise that already had solid classics like MW2 under its belt. Unbelievable story, great MP, superb maps, loads of great new game modes, etc. Totally exceeded all expectations. Tended to be very much a case of diminishing returns with the sequels, but BO4 was reasonably innovative with its inevitable foray into BR.

Treyarch did the same with COD: WaW, it was a pretty innovative title that brought a lot of new features we hadn't seen in a COD title before that point....but even aside from that, the MP mode was inherently excellent, as was the campaign.

But COD has badly fallen victim to this vicious cash-cow business model and new releases being demanded on an annual basis. EG, I think most people would put games like Infinite Warfare and Ghosts in the "didn't really add anything" list for certain. (in terms of MP, very few people play the campaigns to be honest, IW had a fantastic one alright that had some new stuff, AW was also pretty good, but COD titles are pretty much exclusively judged on their MP offerings for quite some time).

A few of the latest CODs also seem more interested in being all things to everyone too, with the aim of generating every last possible $ through lootboxes and in-game purchases. Whacky cartoony skins, pink glitter unicorn guns, etc..... wanting a slice of the Fortnite pie.

COD WW2 was one I was looking forward to personally but I thought it was very weak - story mode was poor, MP was alright but had painfully generic maps, the game itself just didn't capitalise on its WW2 theme at all, and the wacky customisation was really offputting.....guys running around in Leprechaun outfits and dressed like old poets, put me right off it. It wasn't that it was a WW2 title. It was that it was a poor WW2 title, but some people interpret that as "you can't win either way" as oppposed to "it just wasn't that good of an effort".

There's still plenty of life left in COD as a franchise. People don't get jaded of it because it's the same old theme, or that they demand some sort of radically different scenario with every offering....it's simply because many of the annual releases are the same old MP shooter formula with little innovation on offer, regardless of whether they're set, and just aren't that exciting. They just exist purely to make money and of course, Activision are flogging the horse as hard as they can as well, a little too hard.

It's an inevitable problem when you set a target to release a new game every year. You would get exactly the same problem with other major franchises like Halo or Battlefield if they took the same approach, you might get some interesting results, but equally you'd also get unremarkable results that would probably do little to set themselves aside from their precedessors. (case in point, Battlefield Hardline)

Fifa or whatever are slightly different I think, as each release reflects real-life changes in rosters, teams, kit, and all that, which is important to some people.