https://wccftech.com/playstation-5-120-fps-native-port/

Previous gen games require full native ports to enable 120 FPS gameplay on PlayStation 5, the Rocket League developer confirms.

Speaking with Eurogamer, Psyonix confirmed that Rocket League did not get a 120 FPS option on PlayStation 5 as it did on Xbox Series X and S because the game would require a full native port, as opposed to the minor patch needed for the Microsoft consoles.