Because increasing the display's frame rate makes zero difference on a S&H display for movie playback unless one of the combinations has jank: If you go from viewing 24 fps content on a 60 fps display it's janky, going to a 120 fps or 144 fps display fixes that jank because frames divide evenly. If you watch 30 fps content, it looks almost exactly the same on a 60 fps and a 120 fps display (but going to 144 fps would introduce jank).