Why is my iPhone 13 Pro Max not starting games at 120 FPS?

I bought an iPhone 13 pro max, where there is support for 120hertz and downloaded games, there is also support for 120 fps, but only the game does not start at 120 fps why???? Wrote to the developers that the problem is you! Says that the device itself limits 120fps !!!! Meaning? Make a 120 hertz screen if it doesn't support it in games.

[Re-Titled by Moderator]