My guesses :
On Series X, you're lock to 30 FPS with RT and 60 FPS on performance mode. So if you increase FOV (meaning more "things" to display on screen at the same time), the console will no longer be able to run the game at 30 FPS and 60 FPS with acceptable drops and you certainely have to decrease some things somewhere else (or play with Ray Tracing at 20-25 FPS or Performance mode at 50-55 FPS, which is out of question, I suppose).
Unlike PCs, where it's to players to decide which % of FPS they are ready to sacrifice for increasing the FOV. Nothing prevent you to play at 5-10 FPS with all the settings at max on a mid-end PC