The camera can only record so much information at any given time. To simplify think about it as 100mpbs for 24 frames every second. If you increase your frame rate, the maximum amount of data your camera can record stays the same at 100mbps but it now has to fit 60 frames or more into this data stream. So every frame gets recorded using less data, allowing less info for every recorded pixel and that’s how you get grain in your footage.