Typically the higher the performance, the lesser the quality. Same can be said in reverse. To make a scene look really good takes a lot of processing power and that lowers framerate. Alternatively, to get to the advertised 120hz, lowering the picture quality frees up that processing power and will raise the fps. Consoles only have so much processing power so they can’t consistently do 4k 120hz. Some less demanding games could probably do both but newer titles can’t.