What tech does each game use? 

What kind of strain does each put on the hardware.

Be specific.

FPS may not dictate what is more demanding on every occasion. But then you'd have to wonder why a game looking like Doom doesn't run like a game like Farcry 5. If it's that demanding.

You just proved you don't know much about this field with that post, some games run at 30fps because they don't need 60fps DQXI is a turn based rpg where as Doom is a fast paced shooter where reflexes and response matters. FPS doesn't equal more demanding.

Conversely, if 60FPS was possible I see no reason why they wouldn't do it, since it doesn't have any downsides.

better lighting, textures, animations, particle effects, shadows, physics, A.i etc...

Oh really? You missed the point.

If Doom can run this smooth on current consoles, it's not because it's "so much more advanced", it's because it's not straining the hardware as much as you think. Otherwise the dev would have settled for 30fps like most other devs. 

Because the's no need for example is a game like chess doesn't need high level physics why add them.

You've got a lot to learn on how games are developed and hardware operates, DQXI is 30fps because it doesn't need to be 60fps.

Physics are different from FPS, it's much simpler to lock a game at 60FPS, especially if it's not demanding on the hardware.

DQXI is 30 fps because ressources are needed elsewhere for different reasons. Streaming assets, loadings, how much is kept in the background at all time, etc. Not all aspects of a game are visually quantifiable.

You don't seem to understand the point it would make no difference, FPS is for response time so games that don't require fast reflexes and responses won't find any benefit from them that's the point you may as well add physics to the chess game while you're at it because it won't make a difference.