Because by the 7th gen every FPS was a consolised gritty brown and grey military shooter where John Marine takes down the terrorists with his arsenal of 2 weapons max and the incredible ability to heal from bullets just by hiding in a random spot like a pussy for a few seconds.
People got tired of this eventually (expecially since they were all still sold at full price)

Only stuff like Far Cry or whatever managed to survive, and you know why? Just look at what companies are focusing on when promoting their new games these days
>OUR NEWEST GAME FEATURES THE BIGGEST MAP WITH THE BIGGEST FEATURES THE BIGGEST SIGN POSTS THE BIGGEST GRASS THE BIGGEST YELLOW PAINT THE BIGGEST PRONOUNS SLIDERS AND ALL THE CONTENT WE MADE SO MUCH CONTENT IN THIS GAME ABOUT THE CONTENT WE HEARD THE FEEDBACK FROM THE GAMERS™ AND WE WANT TO OFFER THEM THE BEST AND LARGEST CONTENT THEY'VE SEEN YET

All those great FPS of the past (Half Life, FEAR, Bioshock etc.) were not big games, they were short highly curated experiences made with people who had a clear vision in mind that wasn't just to sell slop.
The problem is that we also got fed with linear soulless slop sold for $60 for so long that people thought going Far Cry was the answer, it wasn't.
So now either your FPS game is some F2P Live Service multiplayer shit, some open world slop or a boomer shooter trying to ape Doom and Quake
There's no inbetween, no space for a new Half Life or Bioshock, no one in the right mind would put all that effort for a non open world singleplayer FPS, and even if they did, that mindset of "I have to get my money's worth" is still there so it would sell like shit anyway.